Second on Magene from ebay, great but slow ship @$~15, BT or ANT+ and will do cadence OR speed. Needs bike computer or phone display.
Metronome is ~$10 and helps you pace, needs no display, attaches easily with cheap phone mount.
All the 0ring mounted stuff is prone to fall off , so cheap is the way to go
Wired sensor / computer is pretty workable, too.
~$150 ante to get you into the bike computer/GPS game, but that really is the way to do it, as a tool you will really be using.