About Sane Magical Prices
First, Sane Magical Prices does tell us why they aren't priced.
Manuals and tomes +2: If these come up, the GM is probably giving them to the whole party as a "level up" type thing
Essentially, the author is interested in items that would usually be sold - Tomes are hardly the case since they would probably already be used, and buying them isn't the common way to get them.
Note that the guide you mention is not a Supplement or official in any way. It's a house rule by a random guy that made a really good and well accepted job.
Guideline on Prices
As you said, for very rare from DMG, prices go from 5k to 50k gp. In XGtE (p. 126), the prices for very rare items are listed as (1d4 + 1) x 10k, so still max 50k, but min is now 20k. For consumables, that price should be halved - Tomes are not usually handled as consumables since they give a permanent ASI though.
There are more details on how to actually buy magical items in both books, but for pricing, these are the values.
I agree with Sane Magical Prices' opinion on Tomes. I would give them as rewards for someone too far behind in stats (e.g. someone that rolled too low) or, at best, to some MAD class. Allowing players to buy them could take away the whole thinking when leveling up and what feats are worth to get - essentially, when you give free ASI, you are actually giving a free feat.