TD has hit the nail squarely on the head above. I'll chime in only because in the thread linked by David-G I mentioned that Henry F. Miller was on
David Burton's list of rebuild-worthy pianos, and you've asked how much would you be looking at to improve the piano. As BDB indicated, that can't really be known without a hands-on inspection (which is always a good idea anyway).
There's no telling what's causing the tonal issues you describe. It's probably the combination of a number of things, potentially even include a dead or dying soundboard, which would be a very costly replacement. You're probably looking at a minimum of several thousand dollars worth of work (rates vary depending on where you live), and potentially much more.
Anyway, back to the notion of HFM being on the list, there are only two paths that would justify the investment:
Sentimental -- if the piano holds a lot of sentimental value, for example it's a family heirloom, then it can be worth the investment (in fact, personally I consider this the best rationale). This could even include simply falling in love with a piano, but the sentimental path is not one where "value" or cost are a consideration, because the plano in question may never have a monetary value that equals or exceeds the cost of its improvement.
Retail investment -- someone who is familiar with the piano market and does this sort of work may recognize that the rejuvenated/restored/rebuilt piano can be resold for a price high enough to justify their efforts to improve it.