Low enough to make gravity jealous: $9.99 (Banggood). Material: Metal, CC: Output current adjustment, Output Ripple: ≤50mV, -IN: Input negative, VO+: Output positive, Working Tempreture: -10°C-75°C, CV: Output voltage adjustment, VO-: Output negative, Output Current: 20A(max.) / 15A (suggested), Color: Silver, EN: Enable / low voltage level shut off output; suspension / high voltage level is effective, Input Voltage: DC 6-40V (10V to 40V is suggested), When input and output is common grounded / no-load CV output constant voltage / CC output constant current, Output Voltage: DC 1.2-36V, Efficiency: 97% (24V to 12V / 20A), Size: Approx. 60x 52x 27mm / 2.36x 2.05x 1.06”, +IN: Input positive, Wiring Method: Terminal, Short Circuit Protection: Self-recovery (cannot short circuit for a long time).
Updated:
Banggood
Warehouse in China.
Link: DC 6-40V To 1.2-36V 300W 20A Adjustable Step Down Module Board
Price: $9.99
Additional Banggood coupons:
1. First page
2. Second page
Aliexpress
There’re some similar (or not) items on Aliexpress
You can use link bot to get even lower price:
Click here to search by yourself. Here are coupons that you can use.
Price history and price alert
Do you need price history chart (for all/most countries) for DC 6-40V To 1.2-36V 300W 20A Adjustable Step Down Module Board? There we go. The lowest price was: $5.99.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood and Aliexpress (only if product name will be the same as on Banggood and I will post it on social media though, so it’s more like Banggood price alert)).