My main platform is eight foot by eight foot. I have some anti-stress rubber mats on the floor underneath to keep the wood dry. The mats I am using have holes in them so when it does rain and gets into the garage it can dry quickly. These are $20 for a 3x3 mat. After that was done I added 2 layers of plywood. One layer runs vertical and the other horizontal. Lastly I have carpet on top that is about $.45 a square foot. This is usually replaced every year.
Box squat box
I did not make this but as you can see its pretty simple. A bunch of 2x6's alternating directions each layer. Then a square of plywood on the top and bottom. This is nice and study but it is a pain to move.
This little platform started out as a place for our shorter lifters to put their feet when using the lat pulldown but we found other uses as well. We now use two as a platform for handing off when benching and we also use them to elevate our glute-ham-raise. These were made with scraps we had left and are one of the most useful items.
If you are looking for a bigger bench press then board pressing should be in your recipe. We use a 1/2 board all the way up to a 5 board. Pretty simple to make and again very cheap. If you do not want a lot of boards lying around make a one, two, three and stack them if you want a four or five board. Make sure you use 2x6's and not 2x4's they are easier to control and easier to keep in the middle of the bar.
Most of this stuff is pretty simple but can save you some money in the long run.