Did Crowley welch on his contract?

It's difficult to know if it was a real contract. If it WAS, then yes, he did welch on it.

The episode in question is Survival of the Fittest. At first it appears like they are making a standard deal, as the script shows:

CROWLEY I can't deny I long to see those two digested once and for all. You have a deal. I suppose you want it in writing?
DICK ROMAN I don't kiss on the mouth.
CROWLEY Your loss. I just so happen to have a standard rider... [he takes a scroll out of his jacket] right here.
CROWLEY unfurls the scroll, which is very long. DICK ROMAN grimaces. CROWLEY holds up a magnifying glass.
CROWLEY I do so like this part. Don't you?

However, note the actual signing of the contract:

CROWLEY "Should the party of the first part…”
We see a close-up of the scroll. It appears to be written in Latin and mentions Sam and Dean Winchester.
CROWLEY “…fail to inform the party of the second part of his intent…"
DICK ROMAN Pause right there. Correct me – that should be "party of the second part vis-a-vis party of the first part," 'cause we just amended clause [CROWLEY mouths the words along with him] 314-sub-a. That's right. You should do this professionally.
CROWLEY Splendid. So, "Should the party of the second part…”
// other dialogue //
CROWLEY "...in which case, the party of the second part forfeits all rights to Canada ad infinitum." I think we're done here.

If you read it, you can see that if Crowley betrays Dick he forfeits all rights to Canada. That's it. It's not a blood contract, there's no threat of death or anything like that. This is why he is happy making it. Later on, with Sam and Dean, he shows he's on their side (for the time being at least):

SAM So what did he offer you?
CROWLEY A fair deal. In exchange for giving you the wrong blood. It's demon, but is it mine? [pause] It's my blood. Real deal.
DEAN And why should we trust you?
CROWLEY Good God, don't. Never trust anyone. A lesson I learned from my last business partner. [He looks at CASTIEL.]

However, ultimately this is all moot, as neither party was sincere in the contract. Shortly after, Dick Roman has the following conversation with Susan:

DICK ROMAN Great. Susan, do I look like a fool?
SUSAN Not in that particular body, no.
DICK ROMAN Good. Have I told you there are three rules to contract negotiation? Bring breath mints, get it in writing, and have a plan for when they screw you. Go to the freezer.

Conclusion

It's hard to know if this was even a real deal. Both parties were obviously dishonest and both parties expected the deal to be reneged. It seems more likely this was simply a fake deal designed to get everybody in the same place at the same time.

However, if it was a real deal, the consequences of reneging were such that Crowley was prepared to accept them, in the hope that the Leviathans could be destroyed.

In many episodes Crowley writes his contracts with a huge range of loop-holes. In this situation it was just another Crowley-like contract where there is only one winner (Crowley himself).