sigma guitar model dm 5

I have the exact same model guitar and while I do not know the worth, as I have been trying to find out myself, I can tell you that my mom bought it in the seventies, I think it was like '75. So it's close to thirty years old. Also I know she paid $300 for it when she got it. Sigma is actually Martin. The sigma company was established in 1970 and did not last for but a few years. Tey must be budget versions of Martin similar to Fender/Squier Gibson/Epiphone. Hope this helps at all.