I've always liked the Model 88. The .308 is probably the most popular chambering in this rifle and I would guess that the .243 would come in second; all of which is to say that, in terms of monetary value, the rarer .358 or .284 calibers would be of greater worth than the more common chamberings. The post-64 status, probably afflicted with the pressed basket-weave checkering, does not help the value. Finally, it's very unusual for any rifle to not have had the stock shortened in order to retain the factory length of pull distance when a recoil pad is added. Any alteration to the stock will adversely affect the value of the rifle. But, if, as you say, the stock was not cut when the pad was installed, that's a good thing in terms of collector's worth. Hopefully, the original butt-plate is still around because a replacement might be very difficult to locate.
In excellent condition, with an unaltered stock and butt-plate, the post-64 Model 88 should fetch in the neighborhood of $800.00 to as high as a grand. If the stock has been chopped, it'll lose at least a couple of hundred dollars in value. However, if you aren't a collector, per se, and just want the rifle to hunt with, the added pad, especially if the stock has been cut to make the rifle conform to factory dimensions, will make it more useful and affordable. Ironically I suppose, if you plan on using the rifle yourself and want a recoil pad on a rifle to fit you in terms of length of pull, you'll have to have that nice, original stock cut...:banghead: