I believe this ones work glasses underneath them; but I might be wronghttp://www.i-volution2008.com/index.php?main_page=product_info&cPath=7_9&products_id=31
$270 for Rx goggles doesn't sound expensive to me. Eyewear isn't cheap (or at least decent eyewear isn't cheap). I've never been a 'goggle' guy - I wear a full face, but if I was, I think I might go for their 'Aviator' style over the Spyder. Would have to try them on.There are classic goggles designed to fit over glasses - I have a pair of cheap EMGO like this. Never worn them - have nightmares of glass shards being shoved into my eyeballs.
If price is the primary concern, websites like zennioptical.com do have a small selection of prescription goggles. they aren't the coolest specs on the block but for under $50 i don't think you can expect much. i've ordered a few pair of everyday glasses from zenni and never had an issue. just need to supply your prescription at checkout and wait about 2 weeks.