I've had detectors that cost from $100 to $5,000 and everywhere in between.
A few things to consider:
1. All of them are in the same ball park as one another depth wise. Some have more bells and whistles but all of them in general are right there with one another. The exception are the Pulse Induction detectors in which case, are a great deal deeper (and start at $1k for the bare bones ones). I just took a Minelab eTrac ($1,500) and a Teknetics Omega 8000 ($600) against an older Fisher 1265-X (circa mid 1980s and discontinued a long time ago) and the older machine is still right there with the latest and greatest as far as detection depth.
2. It depends on what kind of hunting. Modern coin hunting? Civil War relics? Old coin hunting? Beach/Jewelry? A little bit of everything? Cause one detector IS NOT going to cover everything. They are just like tools...each one does things a little better and it's always best to have more than one LOL. Modern coin hunting is the easy one...as the coins are newly dropped and aren't deep, and a $200 machine with target ID will be just fine. Old coins are different...they are more than likely still in the ground because of being missed by other detectorists...for either being masked by heavy trash (iron) or just missed. Depth is so so...people think it matters alot but masking and unmasking ability in a machine far outweigh the depth end of it for coins. The Minelab eTrac is a good example of this. It doesn't go any deeper than any of the other machines out there but it can cut through iron laden areas with stunning target ID ability...and thus you aren't going any deeper in the ground but you are picking good targets out of iron with a visual aid...and your old coin count sky rockets. Civil War relics are more of a beep and dig kind of detector setup...depth is a bit more important here but just doing good research and finding new sites will always pay off instead of going back and re hunting already hunted ground. A good low cost machine for this would be a Tesoro Vaquero or Garrett AT Pro. Then it jumps up to the Teknetics T2/Fisher F75...and ultimately, the Minelab GPX series.
3. Resale value. If he gets him a low end unit and goes out and gets discouraged by not finding anything but junk...well it's gonna sit in the closet and never get used. Or you can start him off with a mid priced unit...have good abilities to handle what he is going to do with it, and then if he decides to upgrade, he can sell the one he had for a little bit of money towards it...vs having to basically give the other one away.
Rule of thumb....stay away from the WalMart and Radio Shack ones. Stick to the major name brands (Minelab, Tesoro, Whites, Fisher/Teknetics, Garrett). Check around for local dealers and go play with some of them. If you are near Tuscaloosa, take the drive to see Brian Pennington at Southern Treasures. He is a great guy and a big detector distributor for most of the south on metal detectors. He would do you right on one and help you out on machines.