The Siemens 6ES7321-1FH00-0AA0 Digital Input Module (SM 321, 16x 120/230V AC) typically costs between $120 and $230 USD for new/surplus units, but can range from ~$130 on auction sites like eBay to around $230-$250 from specialized suppliers like ArtisanTG and IQElectro, with some industrial distributors listing it much higher (e.g., RS Online at ~$1800+), so check various sources for new, surplus, or used options. 

To get the best price, compare offers from multiple sources, including industrial suppliers, online marketplaces, and used parts specialists.