i don't have any expensive basses - currently the one that would sell new for the most is an ibanez SR500. that being said, serious photography enthusiasts pay easily over $3500 for a decent camera and lenses, and well over that for a good camera and high quality lenses. guys who rebuild cars can easily spend tens of thousands on their hobbies, and do-it-yourselfers need good quality tools that will also cost more than your bass.
i don't know how your fiance grew up or what her interests cost, but i see a lot of women and some men who do their best to convince, bully, pressure or otherwise talk the musicians in their lives into giving up their musical expenses. the way i see it, if you can afford your regular expenses, and the money you spend is your own, i don't believe significant others should get a say in how it's spent. more important, i can't see why they WANT a say in it. your hobbies, interests and passions are a part of you, and IMO they should support that in the same way you would support theirs. my husband is always stoked to get me or help me figure out the latest piece of gear (he paid for half my SR500 and drove me to 3 guitar centers and 2 sam ash stores last weekend) and i was happy to help him get his new jackson V a few weeks ago, as well as paying for all or part of his 8 string guitar and 5 string bass.