You forgot that some vanilla players like the fact that they can keep a persistent world between vanilla updates. Modders can't do that ATM.
Actually several vanilla updates [such as 1.7, or b1.8] significantly changed the world-gen, so using an older world would result in a very unsightly mess [effectively forcing players to reset, or pull of some insane world-editing to fix the mess].
Likewise its modPACKS that normally lack the persistence- individual mods [with one or two exceptions] can normally port over without too many issues.
(though there is a strong correlation between vanilla dropping persistent worlds, and mod devs doing the same through that update)
About three thousand years ago, a wise man said "everything is worth what it's purchaser will pay for it". It's still true today. I've bought the odd skin pack for various games for the price of a coffee, and I choose to 'pay' for some Minecraft mods by donating to the creators.
Yep- this would make modded MC the most expensive game I've ever owned. Totally worth it considering how much fun I've had with it- very good value for money.