If you're a relatively talentless noob who gigs maybe once a month in a covers band?
And what total value is acceptable?
I'm entertained by the drift of the thread, but back to the OP's question...
I see that you have two Warwicks (Corvette $$ and Fortress One) and that you're thinking of alt-tunings. I'm guessing that you're not the talentless noob you suggest - or are you talking about someone else?
Just from a practical side - and focusing on just the scenario of a bassist in a once/month cover band - I'd say two basses are the MINIMUM (one backup). However, cover bands often need more sound variation (tone or key) and unless your FX & amp can deliver that, you'll want to have instruments to cover that. I can see having four or five instruments at the ready if your band is going from classic rock to contemporary pop (P-bass, J or PJ combo, alternate tuning(s), fretless).
As for total value, I think that's relative to your income from all sources (non-musical and musical) and should be a total for all instruments, regardless of the number (i.e., a couple of boutique basses vs. five or six good basses). As a starting point for discussion,
how about 5% of your gross annual income? I don't mean spend that much a year; I mean total valuation of your instruments at any time. I'm also not including amps, effects and related equipment.
I have a significant investment in guitars, PA and recording equipment also, but if I
only played bass, I think 5% would be a benchmark. (My estimate for all musical equipment for me is about 14-15%)