This may be a long post, but I'll try to give my objective, but somewhat limited, opinion on the subject. This will be a side by side comparison of a MIM and an affinity model.
I own a c. 2015 or 16 MIM p bass special that I bought new from
musicians friend. It was on sale for 750, normally 899. If you're thinking about buying one new, definitely wait until they are "on sale" bacause it happens quite often. The model name means that it has a maple jazz neck with block inlays and a 70's style headstock and PJ pickup configuration. It also has a high mass fender bridge that looks a lot like a badass bridge. I've also recently looked over a couple squier affinities.
This is my p bass:
Sorry it's not an awesome photo, but that's what they all look like, so...
Here's what the pickups and controls look like, stock:
The pickups are definitely the weak spot in this instrument, and it's why I was initially unimpressed with the instrument and why I replaced them. To me, it is disappointing that a bass at this price point features a "noiseless" pickup with a bar magnet on the back. My feelings were that I didn't hate the way the bass sounded with the stock pickups, it was just not interesting.
So, next we have a very similar squier affinity model innards:
Immediately apparent are the much smaller and cheaper pots. I dont have a picture of that whole bass, but it's pretty easy to imagine what it looks like.
The biggest difference between a MIM Fender and the squier affinity models is the care paid to the finishing of the instrument. The mim came to me ready to play, and the thing I was upset with was the slightly sharp fret ends that I dont notice unless I really try.
In contrast, both of the affinity basses I've looked at (one arrived with a broken tuner, so I had the privilege of looking over two identical basses very recently) were fine, but the finish work on both was really sloppy. There was glue in places it shouldn't be, like drips on the body and excess dripped on the fretboard. The back of the neck had obvious tool marks which had not been sanded out.The fretboard was very dry. The frets looked smaller than what is on the MIM as well, they were finished ok, although it didn't look as though they were very carefully polished. The back of the neck was not fine sanded, and the finish was a very thin sealer- thin to the point where it could be called non existent. If you are at all handy and not averse to spending a couple hundred bucks and a few hours on upgraded electronics, the squier affinity could be nearly as good as MIM model for much cheaper.
Objectively, a MIM bass is often a very nice instrument, and looking at the price of a new American Fender, they are a good buy. However, if you want to work on a bass a little and dont care what it says on the headstock, the low end squiers are very good value. The main thing you have to consider is what your time is worth and how much you like fiddling with your guitar. Also, its is unlikely that you're going to have any sort of fair return on the money you've put into it if you decide to sell it.
The math would work out thusly to make an affinity PJ up to the quality of an MIM:
Bass: 200
Pickups: 140
New pots, caps, shielding: approx. 35
New bridge (not 100% necessary, imo): 75
Tuners (I can neither confirm or deny that the MIM tuners are "better," I think you would have to decide over time): 100
Sandpaper and sealer for the neck: approx. 20
Total invested: 570, for a bass that plays and sounds like it cost 200 or 300 more.
Tl;dr: MIM Fenders are NOT bad, especially on the used market. They are priced fairly. However, lower end Fenders are a good value if you can work on your own stuff.
