Ahem... as to the OP; Here in the U.S. (or Colonies) the spray cans of paint that you reference as "not cost effective" actually cover a rather large areas. The secret is to look around the paint section of the local DIY and find an attachable handle for the cans. Like this:
Amazon.com User Recommendation
It keeps your index finger from turning any particular color and aids in applying an even coat. It just snaps on.
(If you choose this method, don't forget to upend the can when finishing and spray until the can is emitting only a clear spray).
As to the "cost effectiveness"; I happen to like the quality of the spray paint finish when done correctly... no brush marks or drips, so I use the cans (indoors or outsdoor quality) to refinish all of the house window, door and garage door frames. Worked great and I only used about 4 at around $7.00 per can for really good quality.
Since you're most likely to be using differing colors (you can also get the cans of primer) I'd think it'd be more "cost effective than the airless sprayer (for which you'd still need to buy paint).
I do like the airless sprayer for large jobs though... I use one to paint the exterior of the house, but it's a commercial quality machine at about $650 (US).