Though I wouldn't buy a tank bigger than 40 gallons from them, glasscages.com has stupid prices on glass. Dunno if buying the cut pieces and getting a $3 60" strip of hinge would be cheaper than just buying the pre-made tops, but worth looking at. I got replacement hinge from BigAl's or Drs Foster & Smith online awhile ago, was waay cheaper than getting a whole new top when my hinge split.
I'm with Mike and whomever else though, get glass. Unless it's a small piece of lexan/plexi/lucite/whatever it will probably bow. The heat from lights takes its toll too, don't forget that. If you go the plastic route you'll have to get a substantial thickness, which may wind up being more expensive than glass anyhow.
A good guy to buy plexi from online (if you do decide to go that route) is delvie's plastics
http://stores.shop.ebay.com/Delvies-Plastics__W0QQ_armrsZ1 I was looking for some totally random colored stuff for a project and he had it in stock and shipped it out cut to my specs the next day.
Good luck!