what will your dad be using to sharpen the knife with? A sharpening system (ie. Spyderco Sharpmaker, etc.), or free-hand on wet-stones etc.?
that makes a big difference.
if he's not using a sharpening system i wouldn't recommend a steel like ZDP to anyone that's not a "knife person." i would even go as far to say that some manufacturers S30V is a little much for non-knife people (and by this i mean people that aren't enthusiasts).
i would stick close to Spyderco's VG-10, or any of Benchmade's 154CM. they've been working with these steels for a pretty good while now and they have their heat-treating processes down on both. they are outstanding choices from each company.
also, where he lives is a pretty big factor. i live in San Antonio, TX and the climate here eats steel for breakfast. the only steel that i don't have a problem with corrosion on the blade here is Duratech 20CV (of Rick Hinderer fame). Even Strider's S30V will start to pit lightly if i carry the knife for more than three or for days without applying some tuf-glide to the blade (either stone-washed, or bead-blasted).
if he lives in a humid environment i would stay away from the tool steels, high carbon's, and the Sandvik offerings (unless you plan to force patina the tool steel, which takes maintenance).
if your father really isn't a knife enthusiast i would think that you would want to get him a low maintenance, great user, daily carry knife and there are quite a lot of great knives right around the price range you are looking for.
a lot have been mentioned here, but like i said i would stay away from the higher grade enthusiast type steels and go for a more maintenance friendly high quality "user" steel that won't rust as easily with neglect, and won't take your dad three days to sharpen.
i'm producing my knives with the Dura. 20CV and it's a shame not more manufacturers use it, it's just about the perfect steel IMHO.
also, stay away from the powdered steels if you don't want the blade to snap with light prying. like said above, you should never pry with a knife, but the reality is that it happens very often, again very often with non-enthusiasts.
try to stick to the smelted steels (ie. 154CM, VG-10, 440 from a good manufacturer). D2, M4 (a lot more costly), and 3V are great tool steels that can stand up to light, to heavy prying (depending on tip strength), but what you gain in toughness, you lose in corrosion resistance. you can force patina D2 very easily but you have to maintain the patina (re-apply once a month of so depending on how heavily the knife is used). this isn't hard, you just soak the blade in vinegar until you see the patina form then your done. you just have to figure out if that something your Dad will want to do. it's a little more difficult to patina the 3V i can be done, but the easiest way to maintain it is to oil it, or apply tuf-glide on it once a week. M4 is terrible with corrosion and usually comes coated, but it probably won't even come into play here because of it's high cost. i don't think there's a M4 knife out there for around $100...but i could be mistaken.
i'm not a fan of lock-back knives so i'll try to be a biased as possible. i've always liked Spyderco's Delica and Endura models, but nor the Endura because of the thicker blade stock. they are great knives, with great steel, and great value...but, as i said i'm not a lock-back knife kinda guy.
however, i do like Spyderco's compression lock that would be familiar to someone that's been using a lock-back knife. the difference is in lock strength. the compression lock is tremendously stronger than that of a lock-back design. the only way for the knife to fail with a compression lock if for the blade to break. the lock will not fail with any pressure a grown man could put on it. The caged ball lock from Spyderco is another super strong design.
again, as mentioned above, the AXIS lock is very strong, smooth, and predictable. i'm not a huge fan of Benchmade knives (personally), but i have always been a fan of the 14204/14255/14210 series knives because i'm a fan of Mike Snody and his designs. adding an AXIS lock to his design was a genius idea and made a tank of a folder. The Benchmade Persidio series is also a great tank of a knife, but you get thick blade stock in the 14205 series. and, the Persidio's are notorious for chewing up pockets and spitting them out, they also show wear very easily on the aluminum scales, as opposed to the 14205 series G10 scales that can be cleaned up with wahl clipper oil (what i use after machining G10), 3-in-1 oil, or WD-40, or really any other common household petrolium based lubricant.....even cooling oil works well. Likewise, the new BM Bone collector series are great knives (VERY similar to the 14205, and Persidio), but you get D2 tool steel for the blade material...which is excellent, but you have the corrosion resistance issues, and maintenance to think about. i carried my large Bone Collector folder here in SA for about a week and just from the humidity, and it being next to my body in my pocket made the whole surface of the blade pit before i added the patina.
Also, another few knives to look at would be the new folder offerings from Boker Plus' CLB line (Chad Los Banos). they are great little (small) knives that are tough (many with frame-locks), quality 440 steel, G10, and easily maintained. but, again because they are small you will get thinner blade stock, so he'd have to lay off the prying. Also, the small Boker/Vox colaboration fixed blades are AWESOME knives! if you want to get something for him to pry with and cut things, check out the Bocker/Vox Prymate
