Amazon offers Prime members the Doberman Security Window / Door Alarm Kit 4-Pack, model no. SE-0137, for $9.99 with free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $5.69.
Home Depot charges the same via in-store pickup.) That's the lowest total price we could find by $2. It includes four magnetically triggered alarms and features peel and stick installation and a 100dB alarm.