I'm getting back in the wine game now - been out of it for a few years. But I still feel the same way as before - wherever your price point is, finding good wine is just a matter of diligence, it just requires more at the low end, and less as you go up in price.
However, I will also say that it totally depends on the wine you're looking for. I don't think you can not look at where the wine comes from, etc. to say that there's a universal price point where there's diminishing returns.
Over the past few years, from what I observed, it was really difficult to find basic, decent French wines for under $15 because of the strength of the Euro. I completely quit trying to buy reasonably priced Bordeauxs because of this. So, I would say with French wines, and especially Champagne, where the number of bottles available under $30 is extremely limited (excluding sale prices), your ROI or where you start seeing diminishing returns might be higher than, say, German Rieslings, where I've found a number of really excellent drinking wines for under $30.
But, for example, a friend (who works for a wine store) pointed me to the J. Lassalle Preference NV Champagne. It's $29.99 and it's a killer deal. I would rather buy a case of this stuff, than 3 - 6 bottles of some higher end Champagnes.
I've always also tended to focus on good years; you'll have to hunt more for better wines in off years. Getting to recommendations, for Alsatian wines, I would still recommend recommend the big players like Zind-Humbrecht, Hugel, Schlumberger, Schoffit. They're consistent in quality and stylistically. For Pinot Noirs, eh - go with a good maker's lower wines. I've been buying Williams-Selyem because I'm on their list and their lower priced wines are in the $40 range and are excellent, if you can find them. I don't drink many Cabernet Sauvignons anymore, or Chardonnays for that matter.
Just off the top of my head, these are wines I've always liked that are reasonable: Bogle Phantom, Allegrini Palazzo Della Torre (haven't had this in a while but it was always consistent), Macrostie Chardonnay (again, haven't had this in a while, but always consistent), Domaine J. Laurens Cremant de Limoux (sparkling wine). If you want to take a step up to the $30-$50 range, I can recommend Delamotte NV Brut Champagne, the aforementioned J. Lassalle, vintage Laurent-Perrier Champagne and Louis Roederer Champagne in good years, Pierre Gimmonet NV Blanc de Blancs Champagne, Miura Pisoni Vineyard Pinot Noir, anything of Linne Calodo that you can find in this price range (the maker is my friend, but I stand by his wines - they're consistently really good), Newton Unfiltered Chardonnay, Williams Selyem Pinot Noirs (again, if you can find them). (Yeah, I like Champagne. I tend to prefer foods that happen to match well with Champagne; for the occasional beef or lamb, I'll definitely drink reds.)
Stylistically, I like balanced wines, so these may not necessarily be your cup of tea. But, like Stefan pointed out - you can buy a lot of other nice stuff for $100.
Good luck!