In an effort to synthesize, we are really talking about a couple of separate options here.
1) A sort of replacement pages subscription, where errata is incorporated into the RB, and printed pages are sent out to subscribers say on an annual basis.
2) A similar service, but you get a pdf of replacement pages that you print at home.
3) An e-RB, for which you get updates on a periodic basis.
I can see some issues arising that would affect how these would be implemented.
1. I have no idea how challenging it would be to do regular layout and proofing for a(n e)RB. One would think this should be relatively easy, after all I can use a word processor, but on the other hand I am no publisher, and I don't know what is involved in the publishing world. Would doing constant layout revisions and proofing constitute a waste of time for the generated return for MMP?
2. For option 1), above, the pricing would likely be fairly high, especially for a one-time payment. Why? Because the errata snowballs. So if you make a big change/addition early in Ch A, this has the potential to cause a reprint of a large number of pages, conceivably a majority of the chapter. If this snowballs for all 5 main chapters, MMP may run into situations where they are having to print large sections of the RB repeatedly, potentially at a high cost. To cover their margins and risk of this, they will have to charge a lot.
3. The e-rulebook would be WAY easier to implement in terms of printing and distribution, eliminating these issues altogether for those that want an eRB. This is a good thing, so long as the payoff for time spent in layout and proof is economical. But it creates a disparity between those who order the eRB (and get regular updates), and owners of the paper RB, who are only getting updates when MMP releases update packs.
4. Besides the above, the ease of adaptability for the eRB causes some problems for selling paper copies. MMP can and does do print-on-demand, as I understand it. But then what do they do about extant copies of the RB? I foresee a horrendous number of versions. So let's say they do a layout that is up to date as of today, and print it and call it RB 3.0. Then in a year, they print an update pack that includes all the errata that have been added over the year. Then another for the next year, and so on. But the updates are not necessarily going to be cumulative - why re-print page D7 in 2019 if it only changed in the 2018 update? Now MMP is forced for a time to include all the cumulative updates when it sells the RB, at least until their stock runs out and they start again. Not insurmountable, but maybe a hassle for them and for those getting into the hobby. Admittedly this is less of a hassle than finding all the errata and stickying it into your RB, but still...
5. Regardless of the options used, I can see the pricing of this being a big issue. We need MMP to make a profit, so they keep ASL going. But I have a hard time seeing a one-time payment, especially for the paper RB. We would be asking them to predict layout and printing costs way in advance. In fact, thinking about it, I think releasing an "Errata Pack" every year might be a better way to go, because they could then adjust price based on production costs. Although I would expect that this pack be priced pretty close to cost - it is errata, after all.
6. What about errata for stuff outside the main RB? Do we get updates for Ch F if we haven't bought the new Hollow Legions (which I think should be called Spaghetti Western Desert, but I digress. Call me, Brian Youse, I'll let you use it just for the mention)? Same goes for HP, RS, etc. What about Chapter H? What about HASLs?
So to the original question, I would pay something like $20 a year for a paper errata pack, more if it was a significant number of replacement pages. For a print-at-home version, I would pay $10 a year, easy.
For a living, regularly updated eRB, I would probably pay around $100 for the full thing, searchable, etc., with a lifetime errata subscription. Less if the updates were not lifetime.