Just bear in mind that any trip you use needs to have the ability to adjust it so you can match the roadbook. Otherwise the money saved on the kit could end up being false economy if you have to abandon an event (entry fee, fuel etc) because you can't match the instructions due to the trip being out to the roadbook, or if your roadbook reader fails in some way. For example the F2R guys have spent years on developing the roadbooks to be as resistant to vibration as possible, and the motors to handle all the banging and shock.
A bicycle tripmeter tends to have set wheel diameters (in my experience) for you to choose so that's a consideration if the max front size the device will cope with is say, 20". You'd need one which can be calibrated to your exact tyre circumference and which can be adjusted on the fly up or down in total distance.
Maybe another way of looking at is that used roadbooks and trips (assuming F2R or MD / RNS, ICO) in half decent condition will always fetch decent money because there will always be buyers so if you did buy new and you don't enjoy it the loss would be small