A realistic first month is a few hundred dollars if you go narrow, and it climbs with mail volume rather than with anything else.
The pieces and typical ranges:
List data. County assessor and GIS sites are usually free and will let you filter vacant parcels by acreage and owner mailing address. Paid land list providers generally run somewhere in the $50 to $200 a month range for filtered pulls.
Skip tracing, which means finding a current phone number or address for an owner from public records. Per record pricing is commonly a few cents to around fifty cents, more for a hand-checked result.
Direct mail. A printed and mailed postcard usually lands around 50 to 80 cents all in, letters more. Five hundred pieces is a small but genuine test, so call it $300 to $400.
Entity and contract review. Filing fees vary widely by state, and paying a local real estate attorney once to look at your purchase and assignment agreements is worth more than any course.
Earnest money on land is small compared to houses, often $100 to $500, and it's negotiable with the seller.
The cost nobody plans for is due diligence on parcels you end up not buying. A perc test, a survey, or a title search on a parcel that dies during inspection is money spent with no assignment fee at the end. Budget for two or three dead ones for every parcel that gets to closing, and write your contract so your inspection period lets you walk.