The springs you say are pricey, just keep hunting on the web, there are some companies desperate to sell so go round making offers to lots of companies I bet you will get some biting! DONT go for second hand with lowered springs they may snap when you stress them, true Renault make the brakes and running gear, but because the Micra is a unique shape the rear springs are Micra only, front springs can be from a Modus or Clio MK3.
check the specs of these Renault springs against the specs of the micra lowering springs for sale and it will give you an idea if they will fit.