If the rifle is a Winchester the value goes up. If the barrel is a Winchester barrels on a Winchester receiver the value goes up. If the barrel is dated appropriately for the receiver the value goes up a notch. If the parts (op rod, barrel, trigger group, stock) are all Winchester the value goes up. If it's set up for a scope like an M1C or D the value goes waaaaaaay up. If it's in the original park that helps also.
A lot of things will change the value. Condition is important. Markings lay the rest out.
A Winchester in good condition with all the major parts being Winchester will bring $1,000 locally. The CMP "collector grade" Winchesters bring big money. An SA in the same condition as the $1,000 Winny will bring $800.
Your grandfather probably won't accept the current value of the rifle and you'll get it and keep it in the family for much less than it would auction for, but it is very admirable of you to want to offer him what he could get for it on the open market. You're certainly a credit to him. Ask him if he'll show you how to handle it and strip it down and shoot it. Try to find a service rifle match and ask him to go with you to spot for you. Let him see his grandson do him and his rifle proud.
http://auction.odcmp.com/auctions/detail.asp?id=886
http://auction.odcmp.com/auctions/detail.asp?id=38
http://auction.odcmp.com/auctions/detail.asp?id=111
http://www.memorableplaces.com/m1garand/m1garandfaq.html
http://www.surplusrifle.com/garand/index.asp
FIELD GRADE: (Fair to Good)
Field Grade Rifles. Most of these rifles have been refinished or rebuilt at least once while in military service and will likely have some parts from other manufacturers. Fair to good condition. Rifle wear will be exhibited by worn and mixed colors of the finish; there may be some minor pitting on the metal parts; wood will be basically sound but may be well used with minor hairline cracks, and many dings, scratches and gouges; wood may not match in color, type of wood or condition. These rifles may have some foreign parts and wood may be Walnut, Birch, Beech or other variety. Rifles do not have import marks. Bores will be generally good with only minor imperfections; the barrel crown may be nicked, and the muzzle may gauge more than “3” on muzzle gauge. The Throat Erosion will gauge less than 5 – well within US Army standards. Do not expect rifles in mint condition in this grade.
RACK GRADE: (Fair)
Rack Grade Rifles are mechanically the same as Field Grade rifles above, but will show more wear on many of their parts and may have more pitting on the metal parts. Wood will be sound, but well used, with cracks, poor fit, and many dents and gouges. Throat erosion will be 5 or more and muzzle wear may be over a 3 on a muzzle gauge. The overall appearance and condition will generally be rougher than any other grade. Fair condition.
SERVICE GRADE: (Good to Very Good)
Service Grade Rifles show less wear and better appearance than Field or Rack Grades. Good to very good condition. Rifle wear will be exhibited by worn and mixed colors of the parkerized finish; there may be some minor pitting on the metal parts; wood will be either Walnut, Birch, Beech or other variety and will be basically sound but may have minor hairline cracks, dings, scratches and gouges; wood may not match in color or type of wood; bores will be generally good with only minor imperfections; the barrel crown may be nicked, but the muzzle will gauge less than 3 and the throat erosion will gauge less than 5.
CORRECT GRADE: (Very Good to Excellent)
Correct Grade Rifles are similar to the Service Grade (above), but will show less wear and use. Correct Grade rifles will have all correct parts for the date of manufacture with 80% or better overall original metal finish. The stock and handguards will be of walnut and correct for the rifle but will have some dings, dents, scratches and marring of the wood finish. Stocks will have the appropriate original inspector's cartouche. The rifle bore will be very good with no significant defects and with a throat erosion of less than 4 and a muzzle wear of 2 or less. Very good to excellent condition. Limited quantities are occasionally available. Prices start at $950. See tables below.
COLLECTOR GRADE: (Excellent)
Collector Grade Rifles have 95% or better overall original metal finish. Rifle bores are excellent with throat erosion under 3 and muzzle wear of 2 or less. Collector Grade rifles have all original parts as they came from the manufacturer. Wood will have a few handling marks and minor dings and scratches. Stocks have the appropriate inspector's cartouche. Data sheets prepared by CMP armorers are included in the butt trap of each Collector Grade Rifle. Excellent condition - little or no use. Limited quantities are occasionally available. Prices start at $1400. See tables below.