Take a look at full-duplex multifunctional printers, many times they are cheaper than standalone scanners. Just as an example, a black and white laser like the Brother MFC-L2820DW should last you a long time.
I got a Brother ADS-4300N to use with p-ngx, works very well and also is way faster than the usual document scanners on MF printers (duplex is done in one pass, for example)...