I wouldn't be able to justify buying any bike over the two that I already have - a nice but modest tourer (Jamis Aurora) and a folder that does the job (Tern C7) - they cover all my cycling requirements bar one, and I'm not sure if there is a bike that I would actually want that would meet the requirement*
I am, however, always looking at bits to buy and bits to replace - given the smaller financial outlay - a lot easier to justify - especially if you can find one cheaper elsewhere to the first place you look (therefore making it a 'bargain')
Realistically, if I think about it and cannot justify it, then I no longer want it, even if I still enjoy 'researching' it.
Price certainly is part of the justification process for me on two counts - a) can I afford it and b) can I justify the purchase at the price over a cheaper alternative. If I pass on 'a)' then I usually fall at 'b)'. When I fall at 'b)' then unless I really feel the need for it, I stop wanting it or the cheaper alternative.
*
I have to admit that the Tern cannot be realistically (practically) transported on busy buses with pushchair and wheelchair users, but it has been successfully transported on less busy buses. I do wonder if a Brompton would fare better but looking at them folded and looking at the space available on a bus, I don't think there's much in it ; the problem is that without taking up a pushchair/wheelchair bay on the buses I use (no decent luggage shelf and hardly enough room for legs and a shopping bag between the seats), the only alternative is having the bike on my knee (together with a messenger bag) and I'm not convinced a Brompton would 'fit'.
