You can look up the resistance of standard speakers wire on various spec charts around the Internet -
American wire gauge - Wikipedia, the free encyclopedia
Unfortunately, I've never been able to find similar charts for Euro Wire. The wire in this chart is Brown & Sharpes or AWG (American Wire Gauge), but the chart has wire diameter, and cross sectional area (mm²), and has resistance in ohms per 1000 feet and ohms per kilometer.
For example, the nearest equivalent to 2.5mm² is 13ga at 2.63mm², which has resistance rating of 2.003 ohms per 1000 feet (6.571 ohms/kilometer).
Keep in mind that for speakers wire you must double the value. For example, 10 direct feet of single wire has a resistance of 0.02 ohms per 10 feet, but for twin lead speakers wire, the value is double at 0.04 ohms per 10 feet. Speaker wire is 10 feet out and 10 feet back for a total of 20 linear feet.
The real
mystery variables are inductance and capacitance per linear unit (foot, meter, etc...). The manufacturer's do have this information available, but they don't give it out to consumers easily.
Here is some information on speaker wire from Roger Russell who is a designer for McIntosh amps -
Speaker Wire
Farther down on that page, he recommends various wire sizes and lengths for various impedance of speakers. Keep in mind that the lower the impedance of the speakers, the higher the wire gauge need to be.
In addition, the cost of wire and cable should be in reasonable proportion to the cost of your system. When your wire and cable costs get so high you are compromising the quality of your equipment, then you spending too much money.
Illustrated -
Which do you think would sound better, a £500 system with £500 worth of wire, or a £900 system with £100 worth of wire?
For most consumers, speaker wire in the £0.75 (bulk) to £3 per meter range is more than enough, assuming it is of sufficient size of course.
But for the front three speakers (Left/Center/Right) the runs are very short, and you won't break the bank with wire in the £3 to £10 per meter range.
It would a a rare and unusual circumstance in which I would recommend wire over that price range.
To some extent, you have to let your wallet and a healthy dose of common sense be your guide.
Steve/bluewizard