Why post a question on local cost in an Australian forum, from the UK? I would imagine the cost would be significantly different in both countries?
Can a ballistice parachute be retrofitted to a PA28 and if so how much roughly?
Probably not, aside from overcoming the loss of structural integrity, the problems are probably hideous. However even if it were possible, in Australia the aero engineering design work, regulator design approval process and actual work to be carried out would cost many times more than your Cherokee is worth, take a life time to achieve and you would probably loose significant payload. Probably best to file that idea in the silly ideas bin?