You're right wanderingwinder - sorry.
In that case, effectively, the pocket copper will boost a single $7 buy to an $8. That's effectively +3 VP, since the alternative is likely to buy a duchy with 7. I'd say, if X were a single bonus and not a turn-by-turn bonus (the latter is just broken), then X could effectively be 3.
It's an interesting idea, but it's pretty close to salvager or mining village.
As for the 6-coppers 3-estates deck, I guess "better" is the wrong word. The deck has much better initiative (turn 1 buy *guaranteed* to be played by turn 2 or 3) in exchange for less of a chance at a $5 card and much stronger dependence on the first hands' cards. In effect, it's not better or worse, just has way more variance. And there are plenty of cards that could make your opponents' life miserable on turn 2 that are not mountebank, such as militia (say goodbye to your $4 or $5 second hand), cutpurse (ditto).