Another question is: are prices largely arbitrary within certain ranges? Is there a contrabass “blue book”?
I don't know what you mean by "arbitrary within certain ranges." Generally plys are cheaper than hybrids which are cheaper than fully carved. Some makers' basses are pricier or cheaper than others. For example, you would be able to get a fully carved no-name Chinese bass for less than an Upton ply. But that doesn't mean the carved Chinese is a better bargain. The Upton ply might be overpriced, but the cheap carved bass might be utterly unplayable.
Just spitballing here, but I'd imagine a decent ply would run $2-3.5. You can get a decent hybrid for $3-6k. And you could get a decent carved for $6k or up. Sure, if you look hard and get lucky, you can find something under those ranges. I'm sure some plys/hybrids will go for higher than I suggested. Only you can say if a particular bass is worth the premium. For example, some folk think Uptons are overpriced. I'm not sure I agree, but they certainly are not bargain priced.
Some basses will be priced unrealistically high. Might have to do with provenance, a pigheaded seller/consignor, or any number of other factors. But in the range you are talking about - <$15k, you will be able to find a TON of really good basses. The difficult thing will be to figure out exactly what matters to you, and how your preferences translate into what you are willing to pay. Also - depending on you ability and intentions, I suspect you could get an AMAZING bass for well under $15k.
I have not done tons of comparison shopping recently, but when I did, I did not feel prices were crazily arbitrary. I might see a bass with a big price tag on it that I didn't personally care for, but I doubt I would say it was a total piece of crap that NO ONE should like. Moreover, asking prices are simply that. Whether in a shop or by a private seller, you are always free to make an offer.
And - if you can afford it - price really shouldn't mean anything. When you are playing out, NO ONE is going to ask, "How much did you pay for that?" (Well, at least not when assessing how you sounded. Other musicians might always be curious, but you can choose how you respond. I am supremely gifted at being able to make an expensive bass sound cheap!
)) Say you can afford $15k, but you play a bunch of basses in that range and fall in love with one for $6k. You should buy that bass, play the heck out of it, and don't think twice. OTOH, if you think the $10k basses are just fine, but REALLY LOVE one for $15k - if you have the cash, spend it. Now, if spending an extra couple grand is going to keep you from putting food on the table...
You still haven't told us where you are, which would open up discussion of the shops available to you. No, there is no "blue book." The best you can do is look at what certain types of basses have sold for recently. If you see mid-1900s German carved basses are regularly going for $6-8k (just spitballing again), that gives you a good reference point to wonder why a particular 1950 German carved is asking $10k. Or $4k.
I play an Engle as well. The most immediately obvious thing you will notice on most basses you try is a different neck profile. Another thing you will want to consider is whether you are going to keep 2 basses and, if so, if you want your 2 basses set up similarly, or differently. When I bought my hybrid, I thought I would sell my Engle. But I found I enjoyed having 2 basses set up very differently - the hybrid for classical and the ply w/ guts for bluegrass/etc. Also nice to have a cheap "beater" bass when you might need to lay it down in a parking lot or something.
Good luck.
) If you need to take a weekend trip to someplace with a coupe of nice shops, that might be time and money well spent.
)
