Ultrasonic flow meters are expensive because they're made for the industrial market. Price does not matter much there, but it has to last 20 years or longer with preferably no maintenance at all.
If you want to do it cheaply, then bu an EUR5 flow meter, saw a piece out of the pipe and install some couplings, and those parts are easily more expensive than the flow meter itself, and introduce possible points of failure / leakage / contamination.