First off, as you say rotors are expensive, so if it’s just a bit of warp/rub then a rotor truing spanner may be the better option.
Regarding the lockring tools, then yes, in theory the same as the cassette, but be aware that if you got your cassette tool a while ago (ie >10years) then there is the chance the inner will be too small and foul on some hubs.
Regarding interchangeability and cost, that’s a hard one. But of all the bits that I’d seek to economise on then a rotor that has a very very long life and so a very low $/km cost (compared to tyres, chain, etc) isn’t where I’d start. But your money, your call. And perhaps depends if you live in Holland vs the Alps.