Finally found the solution, thanks to Sheesh B.
modification is done by editing the price.liquid file :
changing this part :
assign compare_at_price = target.compare_at_price
assign price = target.price | default: 1999
assign available = target.available | default: false
assign money_price = price | money
assign tax_price = target.price | default: 1999 | times: 1.20
assign tax_money_price = tax_price | money
and finally this one :
Thanks