Is there a standard equivalent for effort between distance and elevation? I'm starting to keep track of how far I ride on paper and while it's nice to have the data, I'd like to know how different rides stack up.

I have one path I like to take which is really flat and goes about 30 miles out (I usually only do 10 and then turn around), but I've found this ride is much easier for me than the other route I take which nets me about 12 miles and about 500 feet in elevation.

I saw on another SE that they calculate 500 feet elevation gain into one mile, but this doesn't seem that accurate for a bike.

So, what I want to know is if there is a standard calculation for "effort" or distance between distance and elevation gain, given perhaps average speed or any other relevant factors.
If you really want to measure how much effort you're putting in, you should look into getting a power meter. It measures the actual wattage you output, and can therefore be used to calculate total energy output. However, they are quite expensive. 

The other option is to get a heart rate monitor along with cadence and speed meters which together can give you a reasonable estimate of how much energy you used.  This ends up being much cheaper than a power meter, but is less accurate.

These are the preferred methods if you a really want to track how hard you are training. This is because many things can effect how much effort it takes to cycle such as hills (like you mentioned), but also wind, your body position, and the road surface itself.