A router is a tool with far more uses than the simple trim work we often think of. And when you add a router table, the options increase. I also suggest you build your own whether simple or complex. Save your money for what you need. (One plan for a router table consists of a flat surface and a 2x4 for a fence). I recommend an aluminum router plate -- I have yet to find a abs plate that really stays flat.
If you really want a cheap first router table, then it is simple to build one in the style of my first table. However, if the budget allows it you may wish to purchase a good quality store-bought table. These smaller commercial tables can be quickly setup on a benchtop and then stored out of the way when no longer needed. Many commercial router tables include provisions for miter slots/gauges and featherboards.
The most important qualities for any router table are 1) a perfectly flat table and 2) a perfectly straight fence.
DIY Materials: 2ft x 4ft 3/4" plywood (verify it is flat), 2ft x 4ft 1/4" tempered Masonite, Qty 2 4ft 2x2's (straight), Qty 2 1ft 2x2's (straight), and a pair of stable saw horses. You will also want to find a sutable fence setup, the simplest being some S4S oak or poplar (preferably Quarter Sawn) and perfectly straight, aim for about 1x4".
No router plate is needed for this design -- the 1/4" Masonite and 3/4" plywood will fully and adequately support your router.
If need be, 1/2" plywood may be substituted for the 3/4" plywood -- the 2x2's will still provide sufficient stiffening of the 1/2" plywood.
For the fence: cut an opening about an inch-and-a-half tall/wide in the center of each piece for the opening then fasten the the second board at a right angle to the backside of the first board. It is a good idea to slightly chamfer the edges of the hole on the face of the fence so that board ends do not catch. Then either cut some triangle wood braces (ideal) or alternatively use some right angle brackets or angle-iron and use it to further brace the two boards. Use about four of these braces, two near the bit (center) and two located towards the outer ends (but at least a few inches in to allow for the clamping each end).
For the table: cut a hole slightly greater than the diameter of your router's base (say, 1/4" clearance all around) in the center of the plywood sheet. Verify that your router can sit within this hole without any interferance -- if any interference is found, cut away the offending material (remember the router will hang upside-down from this hole, so double-check orientation before removing material). Now fasten the 1/4" tempered Masonite, tempered side face-up, to the top with some 3/4" brads. Nail it around the edges and around the opening you cut for the router.
Setup the sawhorses and measure to determine what distance apart the 2x2's need to be fastened at so that they fall just outside the saw horses. You will then fasten these 2x2's to the underside of the table. You will then measure the optimal left-to-right spacing of the saw horses and use the two 12" 2x2's to fasten to the underside of the table as well. The 2x2's will coral the saw horses and ensure that the table can not slip forward-to-backward nor left-to-right on the saw horses.
The Router Opening: Now remove the subbase from your router and center it inside the hole you cut earlier in the 3/4" plywood making certain that the bottom side of the subbase is resting on the masonite. Now mark out the 2 to 4 mounting holes for the subbase that ordinarily secure the mounting base to the router. Then drill out these holes so that they are just 1/64" wider than the bolts. Flip the table over and drill either counterbores (if using round or panhead bolts) or countersinks (if using countersink bolts). Afterward you should be able to mount the router, without the subbase, to the underside of the table using the new bolt holes that you just drilled and conterbored/countersunk. However, for now just bolt the subbase to the table top and mark the center opening. Now remove the subbase plate. Find the center of this opening and either cut it out with a jig saw or a hole saw of appropriate diameter. If you have another router (or another base for the router) you could leave the subbase bolted to the table and mount a pattern/trim bit in your router to cutout the hole using the bolted on subbase as the template. Once the center hole has been created you can then mount the router, minus subbase, in the table. You may also wish to mount a chamfer bit in your router and slightly chamfer the edges of the Masonite bit opening.
You now have a very inexpensive router table. You can either use a pair of clamps to secure the fence in place OR if you wish you may fasten ONE end of the fence to the table by drilling a hole in one end (just large enough for a 1/4" bolt) of the fence's rear and then through the tabletop (be sure you clear the saw horses and 2x2's). You will then secure this end with the 1/4" bolt and locknut so that it is snug, but just loose enough for you to still pivot the table. With this modification you will only need a single clamp and you can more easily make minor adjustments since only one end of the fence will require adjustment. You may wish to create additional holes in the table so that the fence does not become too angled relative to the front and different offsets from the bit (or with different bit diameters).
Just one word of caution when building cheap tables like the above: Your router will be suspended at quite a good height above the floor. Many fixed base motors are designed to seperate from their fixed bases, which means that it is possible while adjusting your router to literally drop the motor from the housing.
This means a heavy motor crashing to a hard floor (DAMHIKT) which can permanently damage the router motor. If you build this style table you may wish to design a simple 3 or 4 sided box that fits around the router to 'catch' the motor if it ever seperates ... just make sure the box does not interfere with waste ejection.
I presently use a store-bought Bosch router table. If I ever get around to building another router table it will be a very nice freestanding table with integrated storage. This means I need floor space to allocate to it, which I don't presently have. The store-bought router tables are nice and small enough to be easily moved and stored out of the way between uses.