It varies a lot depending on where you get it and what thickness it is. Brand new acrylic with the protective film on it from a hardware store tends to be expensive, somewhat less so from a specialty plastics dealer.
If you're not looking for presentation quality acrylic a junkyard or the like can be a good source. Some scratches can even be removed with the right techniques. A lot of decent plastic gets thrown away by companies in the form old/busted display cases, vending machines, etc, but it's kind of tricky because of all the different types of plastics.
A good trick is instead of buying expensive plastic cement (and there are different types for each type of plastic) just dissolve some scraps of the plastic you are gluing in a solvent such as acetone to make your glue.
http://www.thekrib.com/TankHardware/plex.html