Alienate your friends. You'll receive fewer invitations to life events. Problem solved.
Or else attend everything, spend more than you're comfortable spending, and please others at a cost to you they won't appreciate.
Really, just explain your situation to friends honestly. If you're too embarrassed to admit to being cash strapped, then ask yourself why you'd attend the wedding or birthday party of a "friend" who resents you or judges you.
I think it's better to explain the situation to people in advance because it shows that you're aware of their upcoming life events and do actually care about them. Begging off at the last minute, in contrast, can seem flaky or apathetic.
Send out a mass email to everybody who might be a "life-event risk" or use Facebook. Add something personalized at the beginning such as,
"Hey, I know you've got a birthday coming up"
"Seems like my friends are all getting hitched these days, and I'm becoming a professional wedding guest. Just in case you and [Significant Other] are thinking of tying the knot, here's my situation ... "
Invite them to talk by phone, and I'll bet they'll be quite supportive.
In the 21st century, people put unnatural pressure on themselves to travel for these life events. And their circle of friends (as measured by Facebook) is bigger and more remote than ever.
Really, it can become absurd. Thinking back just a few decades, people wrote letters to congratulate each other. They didn't spend a fortune or spend a week partying. My parents, living in Egypt as newlyweds for 5 years, only had 1 or 2 phone call home each year. Their parents were quite well off, but even they only visited once – for the birth of their first grandchild.
Now that video chat is a possibility, why on earth are you expected to fly around the planet for face-to-face trips? For a dozen or more acquaintances?
Set your own standards. Be candid. Those friends who resent you for sanity can safely be jettisoned.