What is the best/cheapest solution for poorly plastered walls?
The best solution is to get them replastered. However, this is a messy and expensive job as you need to hack off the old plaster first. The expense comes from hiring a plasterer.

The cheapest solution is to cover them with drywall. This will give you a nice smooth wall.

You can glue the drywall directly to the plaster which will work if the current plaster is not too uneven. A better finish will be obtained if you batten the walls first ensuring that the battens are vertical and level horizontally. However, as @Tester101 points out you'll have to "pull out" all your sockets and light switches and have issues around door and window frames.

A better solution would be to rip the plaster off and then plasterboard, again either gluing or battening depending on the quality of the wall. Battening would allow you to add insulation and/or a void for feeding any cables you might want to add (ethernet, coaxial, power) in the future. This is still messy, but should be cheaper.

Sanding is possible, but it will generate a lot of dust and is a lot of effort for indeterminate results. If the plastering isn't too bad you might get a good finish, but you will have to skim the walls again to get a really smooth finish.