Boston childcare prices are insane. I highly recommend forming a babysitting co-op with 2-3 other families or doing a babysitting trade with someone else. This only works if you have at least 1 free day a week to do your child care day.
Basically, you care for your own kid and someone else's kid(s) for one day a week (or more if it works for you) and the other parents do the same on other days of the week. It can be difficult to find people with compatible schedules, and in my experience some stay-at-home parents don't always quite get the urgency of having definite dropoff times and the absolute necessity to stick to the schedule every week, but once you get it figured out it can work really well. You also get to know the other family (ies) well too, which is also really nice.
What part of Boston are you in? If in Jamaica Plain, Roslindale, West Roxbury or thereabouts there is a very active parenting email list and people are always organizing this kind of thing. Other neighborhoods often have local email lists too. You may also have luck posting an ad on Craigslist. I found a nanny share that way.
Which is another possibility: A nanny share is when you and someone else hire the nanny together and share the costs. It's good policy for the nanny to get paid a higher wage for watching 2 kids, but even then if you split it down the middle you save money. (say $22/hour for the nanny = $11/hour each, which is very cheap for Boston) Some people do this every day or just one or two days a week. (I just did it for one day, along with a babysitting trade for another day.)
Some people with bigger child care co-ops also hire a nanny to work with the co-op since once you're taking care of more than 2-3 toddlers it can get a little crazy. Then the cost of the nanny is shared by all, the parent in charge is under much less stress because they aren't alone and can actually go to the bathroom, cook meals, etc., and you have a bunch of kids who really get to know each other and have a good time. It can definitely be a lot of work! but it also can be pretty fun.