So retinAL is slightly more effective than retinOL, in that it takes one less conversion for your skin to utilise it. So technically they are not exactly the same thing.
However, you can find cheaper retinal products (geek and gorgeous for example), and actually tretinoin is much stronger than either retinol or retinal so you might want to try that - but it’s usually only available on prescription.
Like alot of beauty products, most of the price difference is paying for the brand name, fancy packaging etc.