The free rules booklet has all the rules of the game as such. It doesn't have all the mission rules though, which you'll definitely want. So I would recommend getting the rulebook, which is 30 I think? 35?
Since 8th Edition invalidated all existing codices, GW issued a bunch of indices (those are the 5 more books) that have rules for every unit in the game. 2 for Xenos, 2 for Imperium, 1 for Chaos, at 15 quid each. Those came out at the start of the edition to make everything playable at a Basic level. Do you need all of them? Not really, no. The one with your army in it is sufficient. It's nice having them though.
Finally, every army is going to get their own codex, as has always been the case. GW has announced they will release 10 of them this year. That includes more in-depth army special rules, stratagems, relics etc. That may not completely invalidate the Indexes though - GW already said that if a unit is not in the codex (apparently they left out some like librarians on bikes that do not have models) but in the index, simply use the most up to date datasheet for it that's available.
So, the expected outlay? Depends on what you want. If you play Ultramarines for example - rulebook and codex would be enough, that's 65 at current prices (hardback both). Throw in the index, that's another 15.