Seems a bit steep chilli, last pump I had rebuilt (actually was an exchange service) was $400 and change. I believe a brand new stanadyne pump from your GM dealer is under $2000. I think my price was around $1400 CDN with discount. Your diesel shop may be able to get a new pump cheaper than the GM dealer. I cant recall if that includes fuel driver module. And iirc, its around 6 or 7 hours labor by the book.
400k out of a pump sounds amazing, GM used to warranty the pumps up to 200k km because of their problems.
I guess $$$ involved depends if you are doing any other work at the time, obviously need gaskets.
I'd have a real good close look at everything engine related before investing in the pump. Last time I had a pump that was on its way out, I was evading it then popped a head gasket. So now, I couldn't evade the pump issue any longer, so attacked the head gasket issue, sent heads to my local shop for a refresh and found them to be full of cracks. So ordered new heads, gaskets, might as well do injectors, return lines, glow plugs, etc, and the pump. Once it was running great again, the turbo took a dive. So I bought the a team turbo. Then got another couple years no trouble, then I guess we outgrew the truck and needed a back seat.
I guess I am trying to say, a drop in "improved" motor would have been cheaper and easier.
Now that I have no more 6.5, I keep my fingers crossed with my duramax. I'm in way over my head on any sort of repairs it ever might need, I miss the simplicity and economy factor of the 6.5 motor.
Hopefully this hasn't gotten the best of you, and you are still enjoying your summer vacation here in the great white!