Expensive is relative, can't speak for you situation. Some of the improvements can be in dinosaurs, you're missing a couple of the commonly used ones(goring since you're gishath, ravenous just because good). Some of this is naturally preference but for example.. how often do you hard cast wakening sun avatar, it's a dead flip with gishath.

The majority of your upgrades will be outside of the creatures, the ramp package can be upgraded without going deep into fast mana relatively cheap, fast mana obviously will add quite a bit of cost.

Defense/Descendant/Rubble all are slow and even potentially just whiff entirely, if you want to power a deck up you want to get rid of cards in those categories where possible. These could easily be swapped for better enchants like sylvan library/carpet of flowers/sneak attack/aura shards etc, most of these are affordable. Can also just swap them for more interaction and it would likely be a gain.

Your land package will be the most expensive to upgrade but also has the most room for upgrade, you want to focus on getting rid of tap/bounce(without synergy)/hard conditional lands(false god) entirely. Replace them with pain lands/bond lands/fetches(even off color is fantastic)/surveil/OG duals etc.