Fotolia lists their prices in credits. Around how much actual money are we talking here?
It depends. Some are available relatively cheaply ($10 or $20) while others cost over $100. I used it so much, that I signed up for a monthly subscription service, where I get five free images a month for only $5 per image (though I've NEVER used my full allotment of downloads).
However, for most authors, Fotolia offers a 'sign-up' bonus where you get 10 free images simply for trying out the service (which is what Switch originally used). Once that runs out, Adobe has a competing service (the one that keeps flashing up when you access fotolia), which offers the same deal. That's ten full high-def images for free, spread out over time. You can visit the site for free, so pick out which images you'd like and grab the best images for each free trial.
Since I signed up with fotolia, I went from spending around $50 to $100 for each cover, now only around $20 now (including the cover image, multiple images on the back cover as well as chapter header and section break images, as well as custom fonts), so for me, it's well worth the subscription fee, even if I don't use it all.
(By the way, if you want to view the price per image, when you access the image there are various tabs above the listed price. One is the subscription fee ("Monthly pack", which doesn't change), one is the price in credits ("On demand purchase"). The cheapest package of credits is 10 credits for $14.00. The other tab lists the credits necessary when publishing the work (helpful when formatting your final product).