I think it's really relative to where you live... as a pp mentioned, if you are able to be on MDC then generally you are reasonably wealthy compared to the majority of the world. But then our society (talking US/CA here) creates more "needs" which make us feel poorer... plus COL is higher here.
But in the context of just US/CA, I would say 'reasonably wealthy' would mean you can meet all your basic needs (food, shelter), all basic 'wants' (internet, car), and a good number of extras (vacations, dinner out, etc.) without too much concern and with some leftover for savings. I do feel that most people who can afford things like private school, camp, music/dance/etc. classes, etc. would be 'reasonably wealthy' but I also realize many may not be, & may be getting financial assistance, taking large loans, or making major
sacrifices to afford it, so we can't always assess based just on what someone appears to afford.
I think almost everyone would say they need 'just $50K more' to feel wealthy, because the more you have, the more you want, and the more you create extra costs -- i.e. people who can't afford an iPod don't have to budget for the cost of downloads, and if you can't afford a fancy restaurant then you don't have to worry about having a nice dress or new jewelry to wear out...
I do not think I am remotely 'wealthy' compared to the people around me. I regularly turn down almost any invitation to do things that cost money (even $2/pp is out of our budget right now!), we don't have any 'extras' (cable, Netflix, gym membership). But we have tons of food in our kitchen. We have 2 cars that run (nothing in walking distance here!). We 'own' our house (well, we have a mortgage, but one day we'll own it!). We have lots of hand-me-downs for DS and a sewing machine to repair my/DH's clothing. DS has lots of toys and we find tons of free activities to do. DH is on unemployment but we are not receiving foodstamps/medical/TANF/etc. We even have money in savings (which may be depleted if DH doesn't find a job soon...) So I feel content with what we have and happy with the lifestyle we live. Sure, I wish I could afford Music Together for DS. I wish we could eat out once every week or two, or buy pine nuts ($8/bag!!! that's crazy!!), or put flooring in the bedroom (it's just subflooring right now). It would be great to not have to compare the prices of beans to get the one that will save us 10 cents. But really, we have everything we need and some of what we want, so in mind, we're wealthy.