In the Boston area, it's not expensive at all.
If you want to do track meets and run with a club in Boston, GBTC (http://www.gbtc.org) is probably your best choice. They're all about track. Every year they send a team to USATF regionals and nationals. They host a track meet (or more than one, I can't remember) during the year.
For meets, there are a ton. To find them, check out the schedule here.
http://www.usatfne.org/track/index.html
During December and January, there are weekly all-comer's meets at the BU indoor track. These are a great way to start tuning yourself up for bigger meets. You show up, pay ten bucks and sign up for whatever race you want. You seed yourself. They have mile, 3000, 800, 400, 200. There are always at least a few fast guys (i.e. 4:20 or better mile, 8:30 or better 3000m) to race but the heats range in ability from that down to someone who races a mile at 6:30 pace).
If you decide you want to focus on the roads and run Boston (or other races, there are a ton) then your choice of clubs really widens. There are a bunch of road-running clubs in the area. GBTC is still an excellent choice, but there's also a few other big ones.
- BAA (http://www.baa.org)
- L-Street (http://www.lstreet.org)
- SRR (http://www.srr.org).
There are plenty of others, especially in the surrounding suburbs (Greater Lowell, CMS, NSS, Whirlaway) but in terms of sheer numbers in the Greater Boston area, those are probably the biggies. BAA probalby has the fastest guys of any of those 4 though, if you're looking for training partners