I don't think the quote they gave you sounds high. Seems normal to me (we've got dental insurance, but I read the Explanation Of Benefits). And our pediatric dentist won't schedule a child for more than a 45 minute appointment b/c that's about as long as a youngster can handle being in the chair/keeping his mouth open.
My DS also had cavities in his back molars (one on each side). When he was 5yo, DH took him in to get the first one worked on (we started him off at a pediatric dentist b/c that's what I ended up with when I was a kid,and we wanted to skip the middle step). DS sailed through the drilling, filling, and crowning with few issues. We both took him in for the next appointment, and it was miserable for everyone, even with the nitrous oxide (DS was crying too much to inhale it). DS has some sensory processing issues, though, as well as anxiety (gags during regular dental checkups; he hasn't been able to tolerate the bite wings in his mouth, so no Xrays for him yet). We combat the sound of the drill and suction by putting big headphones over his ears and playing music he likes. We have him wear sunglasses to block the bright exam light. However, there was nothing we could do about the feel of the cold water on his super sensitive tooth. They cranked the nitrous as high as they could safely go, they administered extra novocaine...they did what they could (drilled out the cavity, reduced the tooth size in preparation for the crown, and put in a temporary filling). And they referred us to a different dentist who uses different drugs.
This other dentist met with us and DS, and had a hard enough time getting DS to open his mouth so he could see his tooth. He was a cool guy, but he told us DS was a bad candidate for this other procedure b/c of his existing high levels of anxiety and that DS had already had a very negative experience (and whatever it was, sorry I can't recall right now, it doesn't work for everyone anyway). And since it was only one tooth, it wasn't worth the expense or hassle/risk to have DS put under.
So at DS's last dental checkup, the dentist noticed the temporary filling was loose (as in, she touched it with her metal hook and it broke off). They were able to apply a new temporary filling right there, in about 5 minutes (no drilling involved). Trauma free.
I still do all DS's brushing and flossing, every morning and every night, and I'm basically keeping my fingers crossed that we can make it another six years without any more cavities in those molars. Surface cavities don't start out as a big deal, but they can quickly become worse. The deeper they penetrate, the longer they take to drill and fill; if left untreated for too long, they'll eventually require a root canal. Now, my DH just had a root canal (after several days of extreme pain), and he said his root canal was actually better than some of his drill/fill experiences; however, root canals are way more expensive (around $1,000). So that's some incentive for you to get these cavities treated sooner rather than later. I will also say, though, that the older DS gets, the more self-control he gains (and the more SI therapy he's gotten), so he does seem to be growing (slowly) out of some of his issues.