There was recently an interesting post by someone on LimeWire's core developer list who estimated that more than 20 person-years have gone into LimeWire development. You can shrink it to 4-5 years of real work. A good port would take about the half of the time... Now take 4 full-time developers (the average number at LimeWire) multiply it with an average salary of a developer and you'll get a quite large number.

You need more than a few ppl. who would buy it. It is maybe cheaper to buy a JVM license from Sun, do some adjustments and compile it for SkyOS.