Bartosz,

you cannot defeat the economics. You don't buy in high enough volumes, but the "cablemakers" do. So the cheapest solution is to get the cheapest USB-COM "cable" from the market (most probably will be based on Prolific chip) and level-shift DTR and RTS to 5V (here a couple of diodes and resistors, or a transistor, will be enugh, given your "data speed").

But to get cheaper, realize it - changing the "output" function of an USB chip (e.g. replacing the UART by a eight-output latch) won't change substantialy it's market value nor decrease the costs - the USB part of the chip is much much more complex... And the priceincludes also the price of USB-conformity test, membership in usb.org, development and maintennce of multi-platform drivers, certificates from Microsoft, and maybe even a royalty fee for using that ugly USB logo. So maybe you can get something cheaper - without these "extras" - but with the associated risks.
If you take that risk, check out the purely sw solution of Igor Cesko (it's AVR - hope I won't get banned from here for mentioning it :-) ). The homepafe is cesko.host.sk, but that's a free server and their service is quite poor, so try googling fot that name and USB.

Have fun!

Jan Waclawek