I found it through the comments below:
What a shame. Dollars to your aunt's cherry the next thing to go will be the Beach Races - I'd put money on it.
And also here in the comments.
This is not a particularly classy expression.
Expressions beginning with "Dollars to..." (dollars to donuts, etc) are expressing that something is likely in terms of an implicit bet (if I'm wrong, I'll give up something more valuable to you, like a dollar, but if I'm right, you only need to give me something of less value, like a donut) (this particular example has lost some of its meaning with the inflation of the last eighty years).
Cherry is, in this context, a metaphor for female virginity. The tacit assumption is that "your aunt" is no longer a virgin, so her cherry is worthless.