Vintage mower values can be rather wide ranging. Their age, condition, and rarity all play a part. Collector's may pay more, individuals who just like to use old style mowers, less.
Vintage lawn mowers sometimes increase in value over time, depending on their age and condition. This page has information about the value of a vintage Ransomes lawn mower.
Whether you collect or are trying to sell a vintage mower, some research will be needed. Vintage mowers vary in value and are sought for a number of reasons from yard decorations to increasing a collector's inventory.