As already stated the plugs are expensive for the TA, they are Iridium plugs and
should last 60k.
When I had my TA is did start to cough and run rough at around 18K, I swapped the plugs which seemed to cure it.
It isn't particularly hard to change them.
There's only two spark Plugs to change on a Fiat 500 0.9 Turbo (Twin air).
www.fiatworkshop.co.uk
The clip on the Turbo Induction pipe is a Clic R clip, if you haven't the Clic R pliers, just open it with a screwdriver and replace with a large Jubilee clip, otherwise you'll struggle to retension it when refitting.
The plug key needs to be long and thinwalled and preferably magnetic, a Lazer 3682 is ideal.
I would also say as airbox is coming off, it might be wise to replace the airfilter as well if it hasn't been swapped recently.