You have to be shopping in the right place. In rural Texas, Oklahoma and Kansas I know of used vehicle dealers that specialize in trucks coming out of fleets or coming off long term lease.
Right now, here, it looks like under $10,000 is good for a 10-15 year old "work" truck, usually extended cab, with around 200,000 miles. Under $15,000 gets in that age range with 100,000-150,000 miles or newer with even higher mileage. Diesels, V-8s, V-10s, diesels often still $3000-5000 higher for similar age and mileage.
These aren't going to be the consumer models with fancy trim and $15,000 worth of optional equipment, rather base equipment on XL or XLT for Ford, LS or 1LT for Chevy, and most of them will be white.
Cargo vans (3/4 ton to 1 ton) tend to be somewhat cheaper than pickups (or stake trucks) of similar age and mileage. They are harder to find with towing options, as most commercial users of these vans don't buy them for towing, they buy pickups for that purpose.
Sample the inventory at
Green Country Auto which is the first place I look locally when I think I want a good, cheap work truck. I'd venture to guess there are dealers like this in southern Alberta, as the market that buys these new is in oil field service, and the dealers are collecting them to sell to farmers and ranchers on a budget with expectations of adding not a whole lot more mileage over the next ten years, just hauling feed, moderately heavy trailers, and other equipment between their various fields.
