Rent if you can. I've rented them for snowboarding trips and it only cost £40 for a week (plus a few days either side for pick-up/drop-off) to get a £260+ Thule box. Bars were included in that if you needed them, but I've got my own. This was from edge2edge.co.uk – I'm sure I've seen other companies elsewhere in the country offering similar. You don't get the problem of storing the thing the rest of the time either.
Thule are pricey but do seem to hold their value pretty well – if I was buying I'd probably scour ebay for a looked-after secondhand one so if you do need to shift it you won't be too out of pocket. Atera and Kamei make nice ones too, similar prices to Thule.
Have a good think about what you'll use it for – there'll be weight limits on the box (and your bars) so ideally you want the bulky but light stuff up there and the dense, heavy stuff in the boot. If you might want to carry skis/snowboards then make sure you buy one long enough!