The cheapest way to pimp your ride is to win races, scoring free saddles, saddlebags and blinders.
The Witcher 3: Race: The Great Erasmus Vegelbud Memorial Derby
Trigger this quest by checking notice boards; it doesn't become available till later on in Act One: Novigrad.
Novigrad's race challenges are much tougher than those found near Crow's Perch: the course is a lot longer and varies with each race.
You can use standard racing tactics in each of the four races: get in front by galloping, but slow to a canter to refill your stamina bar as often as you can, using narrower sections of the path to help force your opponent to stay behind you.
The first three races take place at the Vegelbud Estate, and you'll have to pay 100 crowns per race. There's no choice offered; you must take each of the three challengers as they come.
After you've won all three races, you'll receive a latter. Read it, then meet Cleaver in Novigrad - save outside his office, so you can try this again if necessary - and accept his offer of a place in a secret street race.
This one can be quite tough because if you wander off-track for more than a few moments, you'll automatically lose - and it's not always easy to see which way the path goes. The trickiest bit is right at the end, when you have to do some narrow turns and get onto a bridge; you'll want to already be ahead of your multiple opponents. You end up back at the starting location, so set a waypoint to make it easier to see when the end of the race is coming.
If you win all four races you'll receive new saddlebags, two saddles, blinders and a huge whack of gold.