The full mechanism is over $400usd, but you can some times find them used. The parts are from
Bikequip, look here. The labor will take you roughly a morning to take everything apart, clean it and put it together. For a first timer on this bike, say 6 hours to a day going slowly. It is not really that difficult. When you buy your ST, pick up a Honda Service Manual - it is the single best tool for the bike.
Since you are doing research.... Here is what you should do to your new bike.
This article is the result of collaboration by Andrew Shadow, CYYJ, jfheath, Max Pete, SMSW, and SupraSabre. Igofar’s list was copied from the first link listed above. Click on the attached file. This one is V.11; in the future, any changes will be noted in the title on the first page as the...
www.st-owners.com
Not trying to scare you away, but for pre '08 bikes, the ECU can die and replacements are a bit less common than hen's teeth. Honda stopped making them and there are no replacements besides a used one from a similar bike (i.e. up thru '07). You can also expect any bike that has not had TLC and regular maintenance (flushing the brake fluid every other year or more often) to need a new Secondary Master Cylinder. Do a search for that (top right this page, click on search and type in Secondary...)and read about it. There is also a great article (blue band atop this page - >Articles>1300>whatever titled article you wish - brakes or SMC, etc.