I looked into a LOT of places for my daughter when she was 3. I was looking for some not so expensive, and not-so-serious -mainly to see if she liked it. Oh, and a VERY patient teacher was needed :-)
Surprisingly, the Lone Tree Rec Center (actually, all the South Suburban Rec Centers) has a TON of ballet classes for the little ones. She took the 3-4 year class (still in it) with Ms. Holly, and it's GREAT. There are only 8 kids in the class, so there's lots of individual attention. It's relatively inexpensive - and Ms. Holly is a SAINT. She deals with the kids seriously, and expects them to pay attention, but she has astounding patience with this age set. They teach classes as young as 18 months. we aren't in South Suburban, so it costs a little bit more, but definitely well worth it. I just signed her up for the fall classes because she likes it (and I like it) so much. (I think her fall class is something like $25/month or something like that?) (And, bonus - you can go work out while she's in class, or just hang out with the other moms outside the class.)
The good thing about these classes is that they are limited time - so you can test the waters without it being too expensive. If she likes it, she can continue with the classes. This particular teacher teaches 18 months through competition ballet, and is from the Colorado Ballet, so she does know her stuff.
I highly recommend it - I will definitely keep her in these classes until she's a bit older, then we'll decide next steps. But, for inexpensive, good, lots of times, great teachers and convenience, you can't beat it!