Mosin is the cheapest to buy and shoot. Rifles are $100 and ammo is very inexpensive and plentiful. It is also the cheapest made and has the worst action of the 3 you mentioned. Regardless, it is really a fun gun.
The Mauser and Enfield are far better made rifles with usually alot of cool markings that you can decipher. Surplus 8mm is still available so ammo is fairly cheap, not as cheap as 7.62.54r, but it's there. There is some surplus .303 available on and off, but don't count on it. Plan on commerical or hand-loads for that.
Depending on what kind of K98 you're talking about...early or late war German, Yugoslavian knock-off, Russian capture, etc you could pay anywhere from $200 to over $500 and have either a very smooth or rough gun/action.
Enfields have pretty slick actions. From C&R dealers No. 4, Mk 1's are around $250 or so.
I refuse to vote as I have all 3.