Don't take my estimate value too literally. It's been a decade since I was actively engaged in collecting Arisaka's. Probably the best way I know of to guage the current value of your rifle is to check the gun auction websites. And see what examples in similar condition as yours (Ground MUM/mis-matched bolt) are SELLING for. Consider ONLY guns that actually have bids.
As I said, to Arisaka collectors, ground Mum/mis-matched rifles are primarily valued as parts guns. To the Arisaka collector, a matching number gun is primary. With condition of MUM being secondary. So the pecking order goes as follows. #1. Mumed & Matching. #2. Matching w/ground/defaced Mum. #3. Mismatched
It makes 0 difference in value if the Mum is present or ground if the numbers are mismatched.
So it is a strange but common thing for a complete/original condition, but mismatched rifle to have LESS value as a complete gun, than it would be worth if parted out.
You can take a $100, Sporterized mismatched T-99 that has 0 collector interest, but has AA sights. Tear it down, sell the bolt, mag assembly, trigger group, AA sight and any stock hardware separately. And come close to doubling your investment. And then use the barrel/receiver as a tomato stake.
With a Matching Number (intact Mum or not), sporterized T-99. You could restore it (if you know what you're doing) And increase the value of the complete gun. But you would have to be very careful to use absolutely original condition, correct arsenal/variation parts. Collectors will spot a improperly done restoration in a heartbeat.
As I said, the seller of your rifle likely priced at parts value. Which is different than collector value. And would put price you paid in a reasonable ballpark area.
If you found the correct numbered bolt, that's an AMAZINGLY lucky thing. I've seen collectors looking for matching bolts. It is a VERY RARE occurrence to actually find it. And it should certainly increase the value of the rifle. (Bolts were not Arsenal marked, so as long as correct configuration, no way to tell which Arsenal it came from)
On the Mum grinding. Yeah, some were ground pretty rough. Some were done very nicely. It just depended on who was detailed to do it. There were no points for quality. All they cared about was getting it off. And the attitude of the person doing the grinding towards doing a nice job didn't matter.
I wouldn't worry much about the value. Values on all milsurps have been on a gradual increase ever since the advent of the internet. The vastly increased availability of info and parts sources has contributed greatly to interest. And therefore value. I remember when $100. would buy a wheel borrow full of Arisaka's at a gunshow. Hang onto it 10 years and it'll likely double in value.