At the bike shop, they purchase a poor-quality bike with what they can afford. When Charlie Brown asks if it runs, Linus replies that "even if the bike walks, we'll be money ahead".
What does it mean?
I take it to mean that regardless of how the bike does in a race, it is so cheap (given their limited budget) that they've benefited by saving money, and are 'ahead' in that respect.
Charlie Brown is always depressed, and his friends often try to help him look on the bright side with such types of optimism (among the bouts of torment delivered, too.)
It means you have saved money, that you've made a good deal.
A British equivalent is "quids in":
'Quid' and 'knicker' are slang terms for a pound. Or you could call it a bar. If you are 'quids in', you have more money left than you expected.