I no longer have a trailer but when I had one I used to take it to an independant caravan mechanic & have it serviced once a year. The running gear etc are very similar to those of caravans. He also would check the floor out for me, although mine was a fairly new Bateson which had a resin floor anyway. I believe that it cost around £90 but it was worth it for peace of mind knowing that all lights, brakes, suspension etc were working as they should. He'd also grease up everything.
Apart from an annual service the only extra expense would really be to replace the occasional bulb which is cheap anyway.