Granted, final arrangements are a bit of a unique situation, but generally I'm fond of saying "I'm not interested in paying that much." I ran this by DH, who is in a design field (and therefore is always trying to sell something people technically don't need!), and he confirms that her hears the same thing all the time, instead of "I can't afford that."
Like others, to me, "I can't afford that" means I don't have money for that. OTOH, "I'm not interested in paying that" sounds a little more like you are in the power position, and have options, particularly the option to go somewhere else. I know when I say "I'm not interested in paying that," if it's not followed by a price softening by the vendor, my next comment is "I think your price is OK, but I'm going to have to shop this because I think it can be better."
It's all about the negotiation dance - these days, vendors of EVERYTHING are interested in capturing and keeping your business. They are more likely to bargain with someone who comes from a position of confidence, rather than apology.
JMHO
Jane