Short answer: from zero.
Stamps on cardboard — free, but zero analytics and customers lose the cards. Works for a kiosk.
SaaS platform — €15 to €30 per month. App, Wallet cards, push, automations, analytics. Connect your POS, customize your colors — done. Less than the cost of 30 SMS messages.
Custom development — from €10,000+. Your own app, your own servers, €1,000-3,000/month maintenance. For chains with 50+ locations.
Most platforms offer 30 days free. Enough to figure out if it's the right fit.