The advice so far has been good. You do NOT use a Y harness when setting up elevons. With a "Y", both servos would be plugged into a single channels, so they could not be operated as the "aileron" half of Elevons.

I know I've seen electronic mixers for around $15, $40 is way too high. With a computer radio FHHuber is right, one servo to aileron other to elevator (usually), then the radio can do the mixing.
Dennis-