Which parts are that expensive?
Its quite a challenge! Datasheets, Frequencies, Components, Tools, Circuits (Design), Protocols, Software (KiCad, Eagle, etc.), MCU's, Formulas's, etc. Its daunting, and there is so much to learn.
But, fascinating!
When I started, it was via the Arduino and MCU's. Although it can get very complicated, i realised the "true" basics are in the analog field. Understand the relationship R/I/V and some of the basics components (resistor, cap, inductor). Look at the basics power supplies, linear vs switch is a must (imo). LC tank circuits are also interesting (and many other). Build a AM Radio!
I don't know what your "level" is. But I would pickup a some kits (for less then < 10USD, ebay, aliexpress), build them and read the circuit.
Understand whats the function of each element, why is it there? A clock, frequency counter, amplifier comes to mind. Then take your AoE and read the fundamentals. Watch some Youtube, like
afrotechmods.
It took me a while before certain "basics" clicked. If you read about a "cap", your like: ok, sounds logical. But a bit later, not so much
For me, its about keep doing it. One day (probably when having a shower) your like : aha! ofc, thats why it works like so... ( and that -so to speak- charges the battery! ;-)
What helped me: open stuff (be careful), check the insides, see how it works. Google the part no. and read the datasheet.
Besides some of the tools (not all are needed or need to be expensive), i think you can get a long way....
As often, two steps forward, one step back
Just have fun, keep it up and don't kill yourself doing so (that would be a huge spoiler
)