So yeah, for co-lo? it makes all the difference in the world.
Other things, like VPSs? it's just a matter of profitability... the margins on VPSs are pretty good if you don't count labor, so it'd be a matter of just not raising my bandwidth quotas like I ought and maybe spending more money on hardware.
Sometimes, there are two substitute products, one where it's traditional to negotiate, and one where it isn't... for instance, if you buy pre-assembled servers? if you aren't getting 50% off list price, you are getting it in the shorts. But, if you are buying the parts used to build said servers? generally speaking, the lowest price on the internet is pretty close to the real lowest price.
This is the primary reason why I do my own assembly; assembling computers is way easier than negotiating Dell down to a reasonable price.
So yeah; this is starting to shape what businesses I want to get into. I mean, co-location only has margin if you own the building. (I'm working on that part, but I don't have an ETA.) - so yeah, if the 'owning the building' deal doesn't work out... well, I'm not going to dump existing customers, but I am not taking more at the moment.