Run of the mill M91/59s are currently selling for $150 from AIM although I think they just went on sale at Big 5 for $130. That's irrelevant if you're not near one though. However, a 1906 date would be extremely unusual for an M91/59 and it would be worth more to someone who collects them. Contrary to popular belief, I really don't, but there are those that do.

In the interest of Gunboards harmony I'm not going to throw a price out there, but will say that it's more than $150. On the other hand, I suspect that what you saw was actually an M91/38 as they tend to run to the earlier dates and few M91/59s are known even as early as the 1920s, much less prior. The M91/38s are not readily available and must be found on the secondary market. I think they're going in the $250 to $350 range these days depending on the maker, condition, etc. Izhevsk is the most common so unless it is also Ka3 (Cossack) marked or in an unusual stock or something of that nature I'd put it on the low end of that range. Either way, definitely worth going back and taking a second look. Hurry, as I type people are trying to figure out where you live and which shop it might be in.