OK, I break all etiquitte rules...if I don't go, I don't send a gift. I know this is bad, but usually, if I don't go it's beacuse I can't afford to: a) travel, give a gift or c) both. And this is the same for friends and family. Occasionally I'll give a 1 year anniversary gift instead (and will point out "I didn't get you anything for your wedding, so I figured we'd get you something for you anniversary) but that is very rare. I
I'm like this with friends and family. So if it was someone I barely knew, I'd RSVP "No" and that would be the end of it. Just tell your friend you ahve prior engagements. Even if it is a concert, you bought the tickets before you got the invitation.