CHINBAR MOUNT AND SKELETON DOOR ON THE GOPRO?
i'm not sure how well this would work for road riding, possibly okay until wind noise became a factor.
i was starting to look at all the mike options then got a proshot chinbar mount  a few months ago. we vlog about dirt riding so the side or top helmet mounts for the gopro meant we were regularly getting our heads wrenched around by passing vines! plus we'd just add voice overs afterward as we had to yell to get any spoken audio even vaguely audible while riding. 
i use the skeleton door for the gopro black 4 and find the spoken audio is quite good below 35mph or so. it's a touch muffled so in the editing software i just kick up the EQ for the higher frequencies. 
although i've never used a mike i'm sure it would be better, but the chinbar mount is possibly the most budget setup of all. and personally i think it's the safest mount too, it lessens the chance of rotational forces in a crash if the camera hits the ground first and helmet impact research shows angular momentum can be as dangerous, or even more so, than the direct impact of hitting the ground.