I have a tight budget , and that means my car gets a really tight budget, even for basic repairs. Most of my upgrades have come about because a OE part has given up the ghost. An example is the windshield, got a big crack in it and swapped it for polycarb. Water pump went out swapped for an electric one, shocks were bad swapped for progress coilovers. I buy used stuff or raw materials and make my own for just about everything else. If you are short on cash this is the method I recommend.
And a method I don't recommend that sort of got me a free car ( and 0 credit rating again).
If you want to get into the purchase price of the car ( and most of the mods in the first few years), I went bankrupt 3-4years ago after I was about to be sued by a shady landlord on a lawyers advise I was told bankruptcy was the best way to go rather than fight.That was with $14,000 worth of credit card debt (which included the cost of the car and cost of living unemployed for a number of months) and another $16k I was getting sued for was all just written off for the lawyer's fee of $900
:o