What is a ballpark cost to freshen up a tired 502? It run's fine, just has 625 hours on it so it's way past time.

625 hrs is not way past its time. Many go 1000 + hrs.
Cost really depends on what needs done/replaced and what parts you do yourself.
I'd say ballpark is 4-8k

I had a pair of Marine Power 502P's that went well over 1,000 hours before they were opened up. I did replace valve springs at 500 hours.

FYI These engine were used year round and cruised anywhere from 3,600 to 4,000 RPM's, and were serviced regularly. I think they had a lot
more to give, but one engine overheated, and had to come apart, so I went through both engines at that time.

Steve

Like Baja said perhaps a simple compression test to determine what, if anything, needs attention. 625 hours is not past its time as Griff said. Barring any compression test issues I'd address the injectors with cleaning and flow testing/balancing.

Along with a leak down test...

Do the heads. You will be amazed how good the bottom end looks like unless it's been abused.

On a single complete rebuild I has estimates form $6000 all the way up to $14000 on 502 mpi . All depends what is preformed. To freshen up top end and if you can turn a wrench. $1000-$1500. Again depends on what you have replaced in the heads.

Receipts on my PQ for both of them to be rebuilt / freshened up was $22k.

well , as mentioned , do a leak down test..
3 years ago, i upgraded my 510 's to 547's, cost was 16-17 k for parts and machine work, i replaced everything from crank to complete valve train everything.
now , this was me assembling motors, spending hours setting ring end gaps, etc,,etc,, also , me installing motors etc...
all depends on how much you want to do, and who is going to do it.

Yep, I get asked prices all the time to "freshen up" bbc marine engines, price can be hard to nail down , huge price difference between re-ringing a motor VS replacing heads, pistons, valve train, cam, boring, decking, balancing, upgrades , dynoing, tuning in boat, exhaust upgrades,etc, etc. And since most 502 mpis are 18 to 25 years old, no telling how rotted heads or what you will find and other stuff is until its apart. Ive seen countless threads on here where guy says he got "ripped off" by some marina or engine builder, I think the majority of the time the guy who gave them initial estimate didn't make it clear enough upfront of what might be involved and then as they dig deeper they find more and more stuff thats not re-usable. I'm sealing up some oil leaks on a pair of blown 502s out of a 30 ft eliminator that were "rebuilt/freshened" 5 hours ago by someone who post here OFTEN, or at least used to. Not sure WTF he was paid to do or told customer he did but its clear the rear cam plug was NOT out recently on either motor so how did he possibly "rebuild" these, hone cylinders with cam bearings in motor? If so, thats some backyard ****,Im not digging any deeper as this customer ONLY wants oil leaks fixed. MASSIVE amounts of blue rtv over top of 1 piece oil pan gaskets, timing cover sealing surface (gen 6 motors), why??? Things looked like a oil explosion, leaking everywhere, oil lines leaking too. These were dynoed too, I cant believe that no one saw all this oil leaking on dyno, tells me, YOU GET WHAT YOU PAY FOR (and sometimes you don't even get that), Smitty