HiTi Dye Sub printers have a cost per 4"x6" print of about 40 cents.
Other printers should behigher.
Here is Steve's review of a HiTi 640GALA:
Note Steve's comments about the HiTi, in the section titled "Low Cost 4" x 6" Printing":
"In comparison the Sony 25-print pack costs $19.95, twice the price per print at $0.80. And inkjet printers can be expensive too: HP PhotoSmart 100 $0.83, HP Deskjet 990cxi $0.88, Epson and Canon are about $0.86. (Per print costs calculated by the cost of ink cartridges and photo paper used to produce an equivalent number of 4x6" photo prints.) And those inkjet prints have no water or UV resistant qualities."
BTW, PC Magazine includes a print cost comparison in their printer reviews, but I don't know how accurate is is.