The Sultan race conversion costs $550 and the car itself can be found 'in the wild' but the lowriders content costs a fortune; You have to buy special versions of the cars and then spend more on making them 'moddable'. Here's a breakdown of some of the costs from a reddit post:
"To start you need to buy one of the base models. You are looking at spending $225,000 - $500,000. Sorry your Voodoo is still rusty even as a low rider.
Next you gotta take it to Benny... the base conversion for your car is going to be around $390,000 to $400,000. This conversion lowers the car and puts some rims on it. Nothing else... So to make a lowrider with the basics is there.
After rolling through the other options - Hydraulics, Custom Paintjob Livery, speakers, trunks, engine mods, and everything else, you are looking at spending well over $1,000,000 depending on the car. I hit many of the high end **** on it and my total cost for a Buccaneer low rider was around $1,675,000."