Well, I did it and invested 750€ (about $909) into a Tunturi E4 ergometer
and Polar heart rate monitor belt
The belt transmits my heart beat wirelessly into the bike and I can set the desired target heart rate on the bike and it will automatically adjust the resistance to maintain the heart rate at the target level which should be between 40-60% of the maximum (for a beginner like me) according to Tunturi's manual.
Of course there are cheaper bikes, but what can I say, I'm an engineer so if I do cycling for cardio, I want to get the optimum effect from it and doing heart rate controlled training should do the trick...
My plan is to start training cardio 6 times per week. Every weekday morning about 30-45 minutes and then on Saturday I'll do it right before sauna, which I tend to do every saturday evening anyway. Perhaps I'll start with 30 minutes and after a month increase it to 45 minutes once I get some basic fitness.
Anyone else here do a heart rate controlled cardio?