Wheelbarrow: 10 gpThe cost of this is based on 2/3rds the cost of a cart in the Delving Deeper price lists. In OD&D, a cart cost 40 gp, so maybe it should be increased to 25 gp ... or you can just let the price stand at 10 gp.
Does not increase speed, but allows an individual to transport their max weight limit at Move 6, or double their max weight at Move 3. Stairs may be tricky, but not impossible: lose control of an overloaded wheelbarrow on a 5+ (1d6.) Slowing down can lower the risk by 1 or 2 points.
I kind of pulled these numbers out of my ass, but there's kind of a rationale behind it. A (western) wheelbarrow basically puts a wheel in place of one porter carrying a two-person stretcher or litter. So, it halves the effective weight. According to this article on the Low Tech Magazine website, the Chinese wheelbarrow should allow full speed regardless of load, with the weight limit based on what the wheelbarrow itself can support; the article claims Chinese wheelbarrows can carry 3 to 6 times as much as a western wheelbarrow.