Steve, When I built mine back in the mid-eighties I expected to be able to do it for $3000, in fact, it ended up closer $7000 and I had no expense on tools as the only tools I used were what I had in the shed at the time.
So far I`ve made 1 set of skins for my home built and 3 sets for my Drifter! each time the cost of materials went up! from memory, the last set for the Drifter cost me between $800 and $900 for materials alone so I reckon if I do another set it will be well over $1000... At the moment, with the value of the Aus $ it would probably cost around $3,500.00 for a set of skins, for the Drifter from America.
You can`t just get any engine and put it on an Ultralight! with high revving engines, you need a reduction gearbox, then you need to be able to drive a prop with it, Rotax`s are popular because they have a purpose-built gearbox, with mine, I used a Robin Fuji 440 engine and chose a timing belt reduction drive.
If you want to build a 95-10 single place Ultralight, as I understand it, you can pretty much choose the materials and the engine you want but I can`t see how anyone would be able to build a safe to fly Ultralight, for $3000.
You might find this site interesting! Rotax engines, Rotax 377, 447, 503, 532, 582, and Rotax 912 aircraft engines, parts & ultralight aircraft accessories.
Frank.