The front:
Depending on how low you want the front depends on what springs your going to need, for a cheap set of lowering springs regardless of height your looking at about £50 upwards (new). Im running G-max on my car 50mm drop (going to be selling in a few months if your intrested) there decent springs. BUT i would advise changeing the dampers to if you want to lower it, its not a must but i would advise it. again your looking at about £40 to fit some front springs IF you supply them. you can get them from various sites on the net and alot on ebay.
The rear:
First of all the saxo dosent use springs on the rear, you can lower the rear of the car without buying any parts as it uses a torson bar, i lowerd mine 70mm on the normal shocks for a while but found the ride very very uncomfortable so i put some uprated shocks on the rear. if your running alloys on your car (not the standard steel wheels) then your going to have issues with rubbing on the rear arch, you got 2 choices really. either modify the arch and grind the lip off so it dosent rub or run some stretched tyres. it dosent have to be a huge stretch, i run 165's on the rear i have vtr wheels. cost to lower the torsion bar is anywere from £40 - £60 depending on who does it.