what differentiates car audio from home audio is that with home audio you just buy the components, connect them, and enjoy. very little effort required other than choosing proper speaker locations (easy) and maybe treating the room acoustics (not cheap, but easy).
with car audio you are the system designer. you don't get an integrated receiver, you get the building blocks for one but assemble them separately - including all wiring, level matching, and tuning. a lot more time and effort on your part. i can spend 3-4 hours just making power wiring and grounding done to an acceptable level. i can put 10-20 hours into an amp install (including designing and fabricating the mounting, planning wire routing, etc.). but i'm not just screwing amps to sub boxes either.
with car audio you are the speaker designer. you don't get a pair of floor standing speakers that have hundreds of hours of R&D in them (with decades of experience), you get raw drivers and the worst possible environment for a speaker (a leaky tin can). you must fabricate appropriate mounts to achieve adequate seals. you must treat the airspace as you would any other speaker - by increasing rigidity, lowering resonance, adding seals, and providing ample absorption (that is water proof). you much choose speaker aiming and mounting locations that may differ from stock if the stock locations are done for form rather than function. you must execute all wiring of the speakers, including crossovers, just as the home audio speaker mfr already did. you must execute all of this with high levels of precision and quality otherwise you'll have failures and lost investment. (figure 10-20 hours per speaker).
with home audio you have a room with typical furnishings offering diffusion and absorption. in a car you have glass all around you - creating many specular reflections. the acoustics of a car are a challenge and need to be considered heavily when choosing placement.
i can say that after hundreds of hours of effort, i have a great sound system with a very small financial investment. but that's after testing dozens of configurations and using equipment i've had for a while (some over a decade). my amplifiers are worth several times more than everything else in the car, combined. and that, i feel, is vital to a successful car audio system. good amplification along with substantial installation effort. i consider 100 hours a good benchmark for a great sounding system (HU, amps, speakers, subs, treatments). less than that, and you are cutting corners and not realizing the potential of the equipment. treat it as if you were designing a home audio speaker, and you'll be alright at any budget. you can't afford to pay a professional 100 hours, and they won't spend that much time on your car anyway. only you, as the DIY owner, care to take that much time making sure every wire strand is perfectly placed, that every wire is routed to the best of your ability, that the fabrication is worthy of showing to the world build logs, of everything, keep you honest.
one huge cost saver is buying used gear. most of my car audio over the years (95%) has been used. and there is plenty of amazing car audio just looking for a home - especially amplifiers.