Didnt have time to read through every post, but here is what I (think) know about sigmas.

-DM or DR 1-7's ....1 being lowest grade woods, 7 being best. early 70's models have sideways M, some later 70's models have sigma guitars EST 1970.

- 1983 moved manufacturing to Korea. Then late 80's moved to Taiwan. Taiwan models are the worst of the bunch, Jap models are best, but some Korean models can still be quite nice in sound.

If there is an S in the Model number, most likely a solid top. The laminated backs or sides are all over the place, you have to look to see if the wood grain in the sound hole is there for solid backs and sides.

- DM1st models are solid tops,the ST stands for it. some have laminated sides and backs, some have solid sides and backs.

- The DR 28, DR 18, DR 15, DR 11 and some others are all over the place. Some are really nice, some are not. DR 41's are really nice if you find an early model I think.

Look up Sigma on Wikipedia for some other good info.
Everyone asking what their Sigma's are worth are not really going to get an accurate answer. The market can only really decide this. Whatever someone will pay for it I guess. The problem is, you need to hear them to know if it's a lemon or a good one. I've heard of early DR1st models sounding way better than DR -7's and I've heard that some Sigmas sound better than some Martins. Me personally, I would take an old Sigma over a new Martin. You can't get that aged wood tone on a modern built guitar. AND there alot cheaper.