There are a BUNCH of Enfield rifles in the US now. They were going for $100 to $150 just 2 years ago, but price has started to climb cause there aren't any more being imported. (overseas supply ran out) If you buy one from a collector, you might be able to get one at those prices. If you buy from a dealer, expect to pay from $150 to $350 depending on condition.
The No4 MkII never saw wartime use, so they all tend to be in GREAT shape. There were also some imported that were new in wrap! The NIW ones I see tend to go for $250 to $350, *IF* you can find one. The used ones in Ex shape go for $150 to $250. Figure you shouldn't pay more than $175 to $200 unless the gun is NIW.
The No4 Mk1* guns can be had for close to the same price, but condition is usually not as good. And there are some collector type guns that will demand a higher price. Figure a nice one that will be a good shooter to be around $150 to $175 depending on condition.
If you KNOW that you are going to buy a 303 Enfield, start looking for good deals on ammo. Look for the Greek ammo cause it is non-corrosive and shoots great. You can tell cause it will have the letters HXP on the headstamp.
Hope this helps,
Gino
Good luck finding one.