What is the cheapest way to get my truck painted purple?

Clean and prep the vehicle properly - hand-wash with pure water, sand down with medium sandpaper, fill and smooth any dents you find, then find a warm and still day, out of direct sunlight. Wipe down with white spirits, mask up the windows and such, then you can use Hammerite smooth rollable formula, which will give a decent enough finish using a roller, and will be VERY cheap and easy! 3 or 4 coats will be best, and ideally a laquer afterwards if you want to keep the colour looking good for years to come.