Is passing on your development phase a possible action? We had a couple times where we had little or no cash left after building and weren't eligible to take any 'freebies' (mail, special route, subsidies).
Yes, you may pass if you wish.
Edit: I'll add in retrospect that in one instance, I could have just broken up a long build into 2 smaller builds which would have avoided the issue (and added an extra depot for end game so it would have been the wiser play).
And the depots would have been worth what you paid for them, rather than one or more being worth less (as subsequent depots on the same build are more expensive.)