There was no real way to *force* W. to marry Lydia - I suppose a shotgun wedding would have been a little ill-bred for their tastes. The only way to bring about the marriage was through bribery. Mr. Bennet, as I understand, is guessing how much of a bribe/dowry it would have taken to make such a man agree to do such a thing, taking into account:

-his debts (unknown, but somewhat guessable)
-the cost of purchasing his commission in the regular army
-the bare minimum the couple would need to have in the bank afterward to have even the beginnings of a comfortable life - basically start-up expenses for a new home, etc.
-and enough more than that to make Wickham feel that he was "getting something" out of the deal, or to make it an offer he really couldn't refuse