Overpaying/Bidding When Trying To Complete A Collection

Not sure if anyone else has this problem, but, I am hot on the trail of completing the US Airmails (mainly used and less the 3 Zeps) -

But I just need a few stamps and I end up overpaying on ebay - I think...

For instance - I just paid $8.27 for a C1. But I have seen them going for under that in quite a few auctions...dumb

Or - even worse - $1.10 for Scott C101-C104 (First group of 4 of the olympics sets)


What tips does everyone have for filling some of the common spots? - For instance, I need Scott# C98A which is a perf 10 1/2 x 11 1/4 version of C98. So far I have 3 or 4 copies of C98 that are all perf 11.

To get C98A - I am thinking I am going to have to keep buying lots of airmail groups/lots until I end up with one...

(Or pay 6 bucks for a MNH which I don't want)

Tips - Ideas - Help! (how do you fill the small holes without wasting money)