Amazon discounts a selection of LEGO City building sets, with prices starting at $5.59. Even better, most qualify for buy one, get one at 40% off. (Discount applies at checkout; add two sets to your cart to see this deal.) Plus, Prime members net free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping starts around $6.)
Note: A few sets are part of Amazon's "add-on" program and can only be purchased with orders of $25 or more.