I don't want to wade into the Hellfire warlock debate specifically, and more generally whether or not Legacy Champion stacks with Merchant Prince. However, may I suggest an alternate reading of the class indicating that everyone might be doing Merchant Prince's capital costs wrong?

Quote Originally Posted by WotC
You receive a +2 insight bonus on your profit modifier (see page 183 of Dungeon Master's Guide II) per class level and a 10% reduction in your capital costs per class level.
Note, this is per level. There is no table entry that indicates this is a flat modifier or a cumulative reduction. Which means that for each level of merchant prince, we apply mathematics. In the SRD, it indicates that when referring to real-world values, normal math rules apply. Capital costs in gp are a real-world value.

Quote Originally Posted by SRD
When applying multipliers to real-world values (such as weight or distance), normal rules of math apply instead. A creature whose size doubles (thus multiplying its weight by 8) and then is turned to stone (which would multiply its weight by a factor of roughly 3) now weighs about 24 times normal, not 10 times normal. Similarly, a blinded creature attempting to negotiate difficult terrain would count each square as 4 squares (doubling the cost twice, for a total multiplier of ×4), rather than as 3 squares (adding 100% twice).
Five levels in merchant prince means cost x (0.90 x 0.90 x 0.90 x 0.90 x 0.90) = cost 0.59, or 59% capital costs not cost x (1- (0.10 + 0.10 + 0.10 + 0.10 + 0.10) = cost x 0.50.

Capital costs will never be reduced to 0, no matter how many levels of legacy champion you tack on. At least by this reading, which while more complex than the normal interpretation, is an equally (if not more) valid RAW.