BTV stands for Baseball Trade Values. It's a website that gives a trade value on every single ball player in the system, where you can put together a fantasticized trade and when posted are given a chance to accept or deny those trades. Although they're not perfect, they give you a good consenses of a player's worth in a trade by reading reports, talking to scouts and managers.
Like every real life trade, you can't just balance the value but you need to figure in the need, undetermined variables and how it effects the 40 man roster. That said I reject about 90% of the Twins proposed trades because they don't meet my criteria. Yet it's fun to research teams to see where their needs are and if they have anyone who'd fit our needs and what it would take put together a reasonable trade.
I know some don't like it but I think it's a nice tool to put together a "in the ballpark" trade suggestion.