The SG series won't have all the bells and whistles of 802.1x support you need. See the Trustsec compatibility matrix for a good listing.
About the least cost of entry is something like a remanufactured 2960C - for instance the WS-C2960C-8TC-S-RF has a list price of US$205.
You can even get Smartnet on it for an additional US$53/year. That's a bargain when you consider you get 8x5xNBD hardware coverage, all IOS upgrades and full 24x7 TAC support.
