There is no spec (afaik) that states how much noise an audio output should produce. Laptops are particularly bad - plastic cases which reduce shielding, lots of high frequency components in a small space, onborad soundcards - again with poor shielding. Sadly it is difficult to tell waht make or model will be troublesome or not without trying. (I have a panasonic that is pretty good - just pot luck)
It could be that you are overdriving the amplifier, particularly if the sound is distorted. You need a line output (not often found on laptops) and a line in at the mixer. Worst combination is headphone out, microphone in on the mixer. If you have to use a headphone output, the volume on the laptop may need to be reduced.
An audio transformer (you need one per channel if you using stereo) isolates the signal earth from the amplifier signal earth, so it can reduce the risk of earth loops that can be a source of interference (series mode interference) Using balanced lines (which it seems your mixer doesn't have) reduces common mode interference.
as for cost, anything from a couple of punds upwards - professional quality (very flat frequency response etc) are very expensive.
For a selection, look here
Maplin > audio
Note that these are components and will need mounting on some sort of circuit oard, and enclosing in a case. You would need one (per channel) of the more expensive ones (about £7.00 or £10,00). The cheap ones are for use in audio amplifiers.
Googling Audio transformers will give a few other sources of supply and more information.