"Plastikote" Fast Dry Project Enamel spray paints are pretty decently priced for primers. Main issue I have is any coloured spray is a gloss finish, which is a nuisance to paint onto. The Black and White have a Flat finish option. I'm currently eyeing up trying the paint pots.
Plastickote alos have a larger range of spray cans in Flat, Satin and Gloss finishes.. I've yet to try them, though will be looking into a Space Wolf Grey or Dark Angel Green equivalent primer sometime.
The paints that come in Airfix kits are often pretty good :3 I've actually been using the Greys to paint my Space Wolves, and they're pretty close to GW Citadel finish. A number of model kits come with paints, though the quality and finish varies wildly and unreliably. A number of the paints that have come with Grafix kits have a decent finish, but the pots smell like bad eggs
I tried once using cheaper paints that come with Paint by Numbers sets. They have a very grainy finish to them, and are really only useful if thats the finish you're going for. relatively ridiculously cheap compared to most other paints, as a £5 PbN will often have anywhere between five and eight different colours/shades to use
Reeves make a good selection of paints, but they have a very glossy finish to them. Its possible to achieve a Citadel finish, but it requires a fair amount of trail and error mixing water and Matt Medium with the very viscious paint. I currently use a couple pots of paint mixed from Reeves acrylic, in DA Green, Dwarf Bronze, Burnished Gold, and Kantor Blue. Dilute with only water and you can almost achieve a Glaze/Ink result, which is interesting/useful.
At the end of the day, the best way to find a cheaper alternative is to go out and buy cheap paints, or cheap products that include cheap paint. Airfix kicks that include paint (and some include REAL Poly Cement glue) are the best bet in my experience, and you have a model kit to either build up correctly, or cannablize for conversion fodder.
Best of luck to you in future painting