10 pts. They should be 10 pts.
No seriously, rhinos used to be like what, 50, 55? I forget but it was around there. (BT codex has it 50, and 70 for the razorback. That's probably what it was) Then, as I recall, 5th edition and the new SM codex came out within a couple months of each other.
For those who don't remember, in 4th, vehicles were a deathtrap, and 5th was a bit of a reaction to that. But at basically the exact same time they reduced the cost by 15 pts. 2 fixes to one problem. And it took a few months, but once it sunk in, yeah, rhinos and razorback were everywhere. Really, the whole game of 40k revolved around killing AV11.
IG codex came around a few years later with 55 pt chimeras. AV12 front, and when you have them 10 abreast, no one cares about the side armor.
And kinda like Flyers, there was a strong suspicion it was done to sell models. I think most Marine players probably had like 2-4 rhinos total. Y'know, about enough to move a few tac squads around, that's about it. Jump packs were common. Later (and it took a while to build) you'd see things like 6-9 razorback lists. So that's something like $200 in little plastic boxes (and personally, I think one of the most boring models in 40k).