I believe it is per ticket. So if you have one ticket for 2 people, it's 17,50€. It is per direction, so if you have a round trip ticket and you want to change or refund both directions, it's 35€.
I haven't confirmed it, but this is what I believe to be true.
17,50€ is what would be what's deducted from what you paid for the ticket if you want a refund before the day of travel.
If you want to exchange the ticket up to the day before travel, you essentially get a refund (less 17,50€), then apply the refund to the current price of the ticket you want (could be another SparPreis ticket at the price that day).
On the day of travel, you can't get a refund in cash. You can apply what you paid minus 17,50€ towards a "voucher" (Zusatzkarte, or additional ticket) at full price for the same trains, route, passengers as the original Sparpreis ticket. You can only use the Zusatzkarte in conjunction with the original ticket.