Value is always a tricky subject.
Value on a M1 carbine is a combination of condition of the whole rifle (like any other rifle) from the standpoint of its value as a shooter . How does it operate,condition of the barrel, trigger , stock and the amount of wear.
Then added to this is the historical value . This would include who made it and if all the parts are correct to that brand and year .
Once somebody starts drilling and taping for different sights most of the collectors don't want to know anything about it unless it happens to have some rare desiriable parts on it.
At the OPs price he has still got a OK gun. If the overall condition is ok he has a great fun shooter , his choice of tossing a optic on it or trying to return it to original type sights (which are excelent by the way).
Generally any 15 rd mag you see around are original type and will work fine . Many (all?) of the 30rd are copies or aftermarket and can be hit or miss.
Even during WWII all M1 carbine ammo was non corrosive and this is one of the reasons carbines generally are still found in good overall shape.
At some point China was making conterfit "LC" ammo and I have heard that was corrosive . Don't really have the facts on that.
Points to ponder- The gun might have sold for $40-$50 or less on the used surplus market in 1960 and the guy who put the scope mount on it was likely very proud of himself . Un-molested a similar gun could be worth $600-$900 today. If Cabellas was selling it for $300+ they likely gave some widow $120 for it