The cheapest I ever built a trailer for was 5000, and it was built off a old 64 Shasta camp trailer that was only 10X6, trailer itself only cost 500. Labor costs? Think if I was lucky I might have made 1000 for the whole conversion.
20ft trailer? If using FRP thats 33 a panel from Home Depot. Do the math, 4x8 panels that's 5 panels per side of the trailer, 2 on front, 2 on back, another 5 on the ceiling. Already 462, plus Thermoplastic rivets, divider strips, trim, caulk, about 600-700 in walls and ceiling alone.
Floor in a trailer that size? At least 250 in materials.
Window? The way I do it it is about 150 for the actual window, plus another 50 for prop shocks, another 80 for Aluminum, mic parts around 30. So what was that 560?
Water system? I run all mine in crimp fit pex, which is one of the cheapest ways to do it that actually works and doesn't break in a month. Parts? About 250. Water heater? Electric Bosch Ariston 4.0 is like 250. 120V shurflo pump, 110 or so. That's another 610 at least.
Electric? Around 300-400 in parts, with cheapo 12" fluorescent lights.
Already at almost 2k in parts alone. Doh, broke the bank lol.
*Note, I have built around 3 dozen trailers, from cheap 5k old school camper conversions, to 25k dollar cargo trailers, to 15k dollar trucks, and everything in between.