There are many cheap options.
You haven't stated how long you want your applications to last for.
As a roof restorer though, i practice using methods that are going to last!!
We have guarantees on our roofing paint ranging from seven to ten years depending on product used. Our preparation is very thorough however.
In your case, the fact you want to go cheap means you don't want it to last too long.
Make sure the existing paint is cleaned extensively and free from loose flakes ect then apply a good quality primer. These can be purchased for around $60 for 20 ltrs and can cover about 100M2.
Any good acrylic exterior paint can then be applied.
Its easy for a beginner to make a mess with an airless spray gun so just use a roller.
Cheers,
Phil Thomson
www.guardianroofing.com.au