Well, if you're quite happy to buy tokens for future trips, you'd simply buy as many as you need of the cheapest tokens that will give you at least as many as you need. In this case, depending on how many future trips you might plan, and how far into the future they might be, you might buy:
550 @ $49.99 + 900 @ $74.99 = 1450 @ $124.98 (11.60 tokens/$)
1875 @ $149.99 (12.50 tokens/$)
3775 @ $299.99 (12.58 tokens/$)

This really isn't a problem for Excel; it's a decision-making one, for which a judgment call is required. Part of that decision-making should probably include an assessment as of the opportunity cost of buying token now & locking up you money in them, vs investing the money in something else. Of course, while you're investing, the cost of tokens might go up, too...

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]