It's been awhile since I've assisted students with this, so there's a chance things aren't the same anymore (they've added things like background checks I think), but converting a foreign commercial ticket used to go as follows:
1: get a conversion to a US FAA private license (the only license they'll honor). It's a simple administrative task that can be done in about 30 minutes by an FAA Flight Standards District Office (free; make an appointment) or a Designated Pilot Examiner (pay money; get instant service). Get a local flight school to assist you.
2: Check your logbook to see if you meet the experience requirements for the US commercial license (FAR 61.129, can be looked up online). Do the required written test (not that hard; just a single test and studying for it is easy with plenty of test prep books available from sites like Sportys.com).
3: Do the practical test for the commercial license with a DPE (couple hundred dollars fee; instant service) or FAA examiner (free; hard to pin down for an appointment sometimes). Again, you should probably arrange the checkride through a local flight school and maybe get a few hours in to make sure you meet the standards. There are no type ratings in the US for aircraft under 12,500 Lbs. so you can use an R22 to keep cost down.
All in all you should be able to convert for under $1,000 US.