Stocks are half a rifles value, if it is a $1200 rifle 100% original, it is at most a $600 rifle with a bummer stock (but useable- say sanded or harshly cleaned). Of course there are variables, sometimes a stock is worth more than an action. If it is just a mismatched stock it could be worth more than the metal, often they are.
If the stock is sporterized, especially in an ugly or impractical way, it can kill a rifles value, as the stock contributes nothing to the value of the rifle. In these cases you should consider yourself buying an action, or the value is the sum of the parts.
I think the best way to look at buying rifles that have significant problems, - all stock problems are significant - is to consider the value of the rifle by the parts that are original. Barreled receiver $100-150, bolt $100, tg group $75, bands $30, add some if the parts are more desirable makes (BSW, early, late), subtract if mid-war non-acceptanced (MB, Sauer sometimes), if buggered up or ugly parts, don't buy it (junk is always junk...)