I don't think YABU OP, you sound very considered and calm really.
It sounds to me as if you're DS/DIL are just going a bit "power mad" and trying to plan the perfect Pinterest wedding for everyone and their dog without realising that actually a wedding is a celebration of your relationship, which doens't exist in a vaccuum. Your relationship reflects the lives you've had and the people who have been involved.
If your friends are close family friends then I don't see why DS wouldn't want to invite them. It sounds sad that he doesn't. I invited 3 sets of family friends on my side and 1 set from my DH's side (he has a larger family and fewer family friends). 2 sets came overall and it was lovely to have them there. They were almost as proud of us as our parents and I was so happy they came.
My parents and PILs both contributed the same amount to the wedding, and me and DH paid for the rest. It worked out about a third each. Parents were involved in planning in that we told them lots about what we wanted and they each had little jobs to do to as they wanted to help, but at no point did they request anything. We asked them if they thought it would be nice if we invited XX and YY and they were all really pleased as had been hoping we would!
I would talk to them again. Make no reference to the money you've kindly given them but just say it would mean a lot to you for your close friends who have seen DS grow up to be there and see what they say. I would hope they'd invite them but if they really don't then I think you'll have to let it go.
TL,DR - YANBU to ask and to hope to have them there. YWBU if you ask repeatedly or throw a strop over it (though you sound too nice for that)