cheapest set up for training with power?

[mai]

if you happen to be using shimano 6800/9000 (or a hollowtech crankset and don't mind having mismatched left/right crankarms) the pioneer single sided pm might be a good choice. you can also upgrade to dual sided later when your budget is not as tight

[zev]

The power tap is now 299 and is one of the most accurate you can buy according to 3rd parties.