Remember that the sheet size is not the entire usable area of the piece, you really end up loosing an inch off of each side for the clamping and sealing margins
About the pum
You can save money by buying a particular air-compressor rather than an expensive vacuum-pump.
Once upon a time you could get the Coleman Powermate (stock number CP0070010), which had a threaded inlet port. This means you can use it as a compressor or a vacuum pump. It also worked great as a small shop compressor or a high quality air-brush compressor. It's also very quiet compared to other compressors of comparable size and pulls more vacuum than most industrial vacuum pumps for a fraction of the cost.
Try to find your plastic locally
The last way to save money is to buy your own plastic locally. High Density Polystyrene is usually the least expensive and easiest to get. You may also want to check on Flame-resistant plastics as well as the ability to recycle them. Recycling will save you money, but be forewarned, many plastics are not taken by most recyclers.