It's hard to find low-cost NAS platforms often because of the mechanism to insert drives and the enclosure add to the cost. An alternative is to use boards like Hardkernel ODROID-HC1 for 2.5-inch drives or ODROID-HC2 for 2.5- and 3.5-inch drives, but each board supports only one drive. The designs are stackable, but you'd need one Linux board per drive, so it's not ideal.
The Korean company has now launched a new model with ODROID-HC4 powered by an Amlogic S905X3 processor and shipping with a case supporting two 2.5-inch or 3.5-inch SATA drives that will start selling later this