This app was mentioned in 5 comments, with an average of 1.40 upvotes
After flatspin999 suggestion, I started looking around for the best app (android) for the speed limit purposes. All but one that I found required the screen to be on and for the app to be in focus ( not in background ). This app (https://play.google.com/store/apps/details?id=org.mrchops.android.digihudpro&hl=en_US) can have the speed limit ( or minimum ) alerts that will still work while the screen is off. The only way for that to work is to set it to window mode.
Like templeofdank has mentioned, Under $50 is going to be tough. If you are willing to save your pennies though, this unit is a really good option.. I have a 1984 XV1000 and am currently using a GPS app on my phone and a decent handle bar mount. I also have a Veypor (a different unit, not the same as the one in my link) which I am currently integrating into a leather holder.
(edit: a bunch of things)
I have just come across this app to keep me within the speed limit. https://play.google.com/store/apps/details?id=org.mrchops.android.digihudpro
You can set different speed limits with different notification and color codes. So far it is good. Anyone tired it? Any other similar app that is worth teying?
DigiHUD Pro. Fantastic for providing useful information while driving. The pro version is very nice. Speed brackets can be allocated, so the color of the text can change based on how fast you are going, which helps keep me honest with myself about how fast I am driving. It also allows a live Google maps view, including satellite imagery, to keep a birds eye view of the area. I used Waze for a while, but I wanted a bit more customizability. I just use Google Maps when I need actual navigation directions. Oh, and I used the icon from Speedtest in place of the original, I felt it worked better visually.