I have a 10 month old and a 5 year old. With the overlap being so small, we just sucked it up and paid. Both parents work full-time, and taking extended leaves wasn't really an option. Baby started daycare part-time at 4 months, full-time at 5 months. But even if both kids were in full-time daycare, it would still be cheaper than a nanny or au pair. We explored the idea of a nanny share with another family, but it turned out to be too difficult to manage for a very small savings.
I wonder what you mean when you say a "fancy" center? From a certain perspective, my kids have gone to small "fancy" centers all along. For me, it all comes down to the quality of care, and that differs for different ages.
For a baby, what I care about most is that she's in a caring environment where she can feel secure, have strong attachment to teachers, and get good naps.
For a toddler/preschooler, I want to make sure she has more engagement and socialization, but mostly around free play--not an aggressive academic focus.
Babies: I want to see that staff is engaged and caring, and that the ratio of staff to babies is small. I want to know that staff turnover is low. When I visit, I look for: babies being held when they fuss, staff getting down on the floor to play and interact with the babies, vigilant monitoring of sleeping or eating areas. I expect that staff will be even more anxious about safety than I am. Baby is currently going to an awesome little center that only takes babies, and it took me a long time to find it. Fortunately, costs in our area are not outrageous ($700-800/month full-time).
Finding good daycare is hard! I visited pretty much every place in town and didn't like much of what I saw. A lot of the bigger centers--even the very expensive ones--seemed chaotic with babies and toddlers lost in the noise. I visited some in-home daycares that were less expensive, but my state allows a higher ratio of caregivers to kids for in-home daycares with mixed ages, and I didn't visit a single one that made me feel good about leaving my little baby there. I visited one that had 6 kids and one caregiver. In a fairly large house, a few toddlers/preschoolers were plopped in front of the TV, one preschooler was sitting in the kitchen eating all alone, another kid was playing alone in a playroom, while two babies were stashed away in a bedroom at the other end of the house barely within earshot of the living room. Having toddlers eating without supervision is an absolute no-go for me. (At 3.5 years old, my very competent kid choked on an apple at daycare. A teacher had to perform heimlich, and EMS was called. If an adult hadn't been right there with her, it could have gone bad fast.)
I'll switch the baby to another small center (the same one my older kid attended when she was little) when she hits about 16 months or so. I like this place because it's close to my work and because the staff are fantastic. They use a loose Montessori style with a lot of emphasis on social skills. While I don't think that following a strict curriculum or forcing early reading is beneficial, I like to see that a center is engaging with the kids where they are and encouraging creative, messy play indoors and outdoors--not just hanging around and keeping the kids occupied for 6-8 hours. Kids this age should be playing!