I don't think it is a problem any more.
2 years ago I would have "worried" about a large chunk of users not being able to download my apps over 3G, but even then I think they went on WiFi regularly enough to be able to get it should they want it.
I reckon *most* app downloads (certainly in the main markets) are done via WiFi anyways, I really don't think it significantly dents downloads these days.
Also, I'd rather have a fantastic looking app that shines on the retina iPad, than a handful of extra users making do with second rate graphics.
Build the best app you can, every day bandwidth is less and less of an issue, not worth worrying about.
(ps, looking through the top charts on the iPad 4 app store, most successful apps are definitely over 20mb, plenty over 50mb and some are in the hundreds, and it isn't hurting them!)