I have to install a 30 am sub panel in a garage. The distance is 200' and it will be in a trench. Could I get away with running 8/3 UF for this or will the voltage drop be too great, meaning I would have to use 6/3 UF. All tha will be in the garage is 5 garage door openers, 4 lights and a dozen outlets. Im trying to keep the wire cost down for the homeowner.

200' is pretty far. I'm not sure how accurate this is, but when I worked in an electrical department at a local home improvement center our wire vendor told us that you should use a heavier gauge for every 75' which would put you at 6/3. Wait for someone else to tell you for sure though. I don't want to misguide you.

Im pretty sure I have to use 6/3, don't think there is anyway around it.

#6CU sounds good!