So the Arduino is USB1.???????????
The FTDI website http://www.ftdichip.com/Products/FT232R.htm says the FT232RL is "USB 2.0 Full Speed compatible". It'll also work at USB1 rates. hope that helps.
Ehh... a decent RS232 Cable off that Lenght will cost the same as that USB1.1 - CAt5 converter.
Also, cabling cat5 (AKA standard network cable) is way more easy since the plugs are very small.
Ah, not sure what kind of cabling you are buying, but for 100 euro you should be able to buy a lot of rs232 cable. For low rs232 data rates (9600 baud or less) take any old wire, for instance that cat 5 you were going to use, and run it. Yeah, there are lots of specs and lots of spiffy commercial rs232 cables with nice molded connectors and shielding and all that, but for low speeds it's unnecesary.
DB9-RJ45 shells are US$2 each, use 'em to make rs232 cables from cat5, or from flat 8 conductor phone cable. I use dozens of such cables for rs232 in my server room.
-j