It will filter out the UV, unless you get that special (and very expensive) plexi that allows UV through. Plants don't need UV in order to grow though. I've always used glass covers, and never had a problem. If you have animals that need the UV, just use screen for a portion of the lid, and put the UV light over the screen part, so that the UV rays can get into the tank.

I use a brand of plexi called "solacryl." Another brand is "OP-4." Basically these types of plexi allow UV light to pass through. They're used in tanning beds mostly.

As for price, it depends on where you look. Some vivarium companies sell rediculously overpriced plexi. If your local glass/acrylic shop has some lying around, it'll probably cost you around $20. Also you could order over the internet. Contact Buck at Suncorp.

Here is a link that might be useful: SunCorp