I drove my Classic for over a year with pop-out. Down to San Antonio and back at least once in that timeframe, 380 miles round trip. No issues whatsover. I flat-out could not afford to fix it, everything else worked fine, and I never saw any ill effects from only utilizing the first four gears.
Gas mileage I suppose? I never paid any attention to it. I still don't.
Drive it. Fix it later when you get the financial opportunity. Obviously take your time and find a really good deal on your choice out your multiple options. From pretty damn cheap rebuilt or other wise excellent conditioin B13 unit, through a B14 which had a minor, half-assed OEM "bandaid fix" over the B13 unit (YES, a B14 can still develop popout), to my favorite yet a bit pricey option, which is a B15/P11 tranny.
And didn't Miko figure out how to get a Spec V six speed tranny to work? I *think* we have a How To/Technical Library thread either on this forum or on the other SR20 oriented forum. I'm guessing that would be a bit expensive as well. Relatively speaking.
Uh...then there's a Par tranny. But only if you are one damned $$$erious, no bullsh*t, racer of some sort.