If you want to put some numbers on it all...
M1A (iron sights, stripped down standard model): ~$1300
M21 equivalent (scope mount, bedded stock, unitized gas cylinder, NM gas piston, NM spring guide, trigger job, National Match barrel upgrade, *optics not included*) ~$2400-$3,000
M25 equivalent (all of the M21 features, but done on a bedded McMillan stock, *optics not included*): ~$3,600-$4,000
These numbers may be off in the current market. I stopped paying attention once I reached the well-traveled crossroads of price and performance.
We have lots of searchable threads here on this subject; I've been down this road myself. I've gotten well underway to a modern rendition of the M21, only to find out that these rifles are money pits in every sense of the word. The deeper you delve into them, the more of a PITA they become. The more you modify them, the less they lend themselves to field maintenance. Keeping everything repeatable is an exercise in witchcraft.
If you want an M14/DMR facsimile, then only that will scratch that itch for you. If you want the best performance, then there are much better options that can be had for the same or less money, and offer much more in the way of long-term sustainability.
They have an interesting history, that's for sure. Lots of people will champion the rifle for its reappearance in the Middle East. Within the battle rifle concept, it performs pretty damn well. Turning it into a sniper rifle is an exercise in frustration. At the end of the day, the writing is on the wall for the M14. Hints can be found in examining the current trend in 7.62x51 rifle contracts.