My philosophy in all things financial is to be smart about where you splurge and where you save. This is especially true for your beauty routine because it's so easy to blackout and spend half your paycheck on Sephora's website or on a weekend trip to Cos Beauty Bar. As long as the product has a clean formulation, you don't have to buy expensive cleansers, moisturizers or micellar water---hence the appeal of French pharmacy brands like Embryolisse and Bioderma.
As for the areas where a splurge is often a good idea, here are my thoughts:
Good serums contain highly concentrated ingredients and work hard to address your skin's specific concerns, so it's best to splurge for a high-quality product here. I will say that one way to get around this is to shop with a brand like The Ordinary where you can buy individual ingredients and build your own serum for less money. But, if you're going to go out to the store and buy a serum, try to budget at least $50-$75 up to $250 if you wish to go to the very high end.
Some say that splurging on an eye cream is unnecessary (at least while you're in your 20s) but I think that prevention is key, so if you can afford it, splurge on a hardworking eye cream to keep the area hydrated and prevent aging. Look for products which contain ingredients such as hyaluronic acid, retinol, niacinamides, and vitamins C and E.
Many cheaper exfoliants rely on physical exfoliation (think microbeads and abrasive scrubs) which can not only be too harsh on your skin but are also harmful to the environment. Opt for a chemical exfoliant that will not only slough off dead skin cells but that will go beneath the skin's surface to clean out the buildup.
Shampoo + Conditioner
This is another product that I often hear you should save on but I find that in most cases, people who say that are only referring to the efficacy of cheap vs. expensive shampoos. Some cheap shampoos might make your hair look better, but often contain silicones which only give the illusion of shiny, healthy hair when they are actually just producing buildup. No matter how well the product works, avoid hair care items that contain parabens, phthalates, and sulfates.
If you wear foundation, you are likely applying it to your entire face which means it is important to have a clean formulation that doesn't negatively affect your skin and that looks natural. Look for a product that contains SPF, works for your specific skin type, and contains little to no toxic ingredients (check the EWG's Cosmetics Database).