I just saw these new refillable cartridges for the Epson 9900:
http://www.macroenter.com/Set_9900_Refill_Cartridges_p/ia-ep-9900-s.htm
Probably would help your wallet to invest in those.
I have to throw some caution in here. Although using refillable cartridges significantly cuts down on ink cost, that forces you to use third party ink instead of Epson ink. (At least as far as I know, Epson isn't interested in selling bulk ink at discounts.) Even if a third party ink manufacturer says their ink is archival, there's several factors to consider.
Epson has spent a lot of time and money developing the inks that they sell today. There has been a lot of independent testing on these inks, both by independent labs, printer owners, and end user print owners. A no name brand ink is a bug unknown, at best, to throw in. Will the third party inks be as archival? Will they fade? Will they produce neutral grays? Will they produce as wide of a color gamut? Will one batch of ink be consistent with a batch of ink made a year later, or would you need to redo your ICC profiles each time you purchase a new batch of ink (even a new batch of a single color, not the whole set)?
Although inks are expensive, they should be a relatively small percentage of the selling price of your prints. Is it worth taking the risk?
Granted it depends on the situation, whether you're looking for the highest quality archival prints that you can get, or if you and your customers aren't concerned about that, and price really is the most important factor. Granted, there are some people using these printers for maps and blueprints that need to be large but won't be used long term, and in those cases, it would probably make sense to cut ink and media cost as much as possible.
