The RF xx 0,3 l/2 are ~25€ (incl. VAT) here, that's ~5€ per empty bottle as you get 0.6l oil with the two (which is ~15€ of the product price when based on the 5L canister that go for 120€ here - YMMV on the pricing in case you live in a country with terminal stage capitalism that didn't ban price fixing).
When you treat them somewhat good they stay refillable for quite a while, the thing that mostly breaks is the rubber membrane in the green cap (mainly when using non-festool oil) which you can avoid by using the white cap for storage and taking care of cleaning the green cap (especially seal). But as you get one green cap per bottle and usually only need one active... you likely already have spares.
But I agree that Festool should step up when it comes to spares and make everything easily available, even parts of consumeables (like the membranes in the green caps).