I've found many references to getting experience for feeding dragons, and I recall the in game tutorial mentioning it. I've searched all over, and I can't find the exact formula for treats to experience. What is the ratio or formula for treats to experience?
Someone with more nefarious internet skills than myself devised a way to snoop traffic out of Dragonvale. He apparently was trying to crack the tough of nut of "breeding magic", but in the process, he found definitive evidence that feeding your dragon gives you 20 xp per food. This means that treats are decisively the best investment of your money because:
I devised an imperfect way to measure how much experience you get for anything. It is not exact, but for the purposes of other comparisons it may be sufficient. The short answer is that this method suggests feeding your dragon gives ~15 xp per food. This backs up the packet snooping tactics used at the link referenced above.
To determine this first I took a screen cap of my park. For this shot I quick grew a large quantity of food to make the resolution better:
Next I fed every last delicious morsel of that food to my hungry dragons:
Finally, I bought something I knew the xp value of. In this case, 2x Shard of Tul - costing 200,000 coins and netting 100,000 xp.:
I then took these pictures, aligned them, and opened them in MS Paint. Paint has an X,Y coordinate marker, which allowed me to compare the bar length between images. I came up with a conversion factor of about 50,000 xp per tick in Paint (which will only apply for my level) based on the Shards of Tul. Given that, I measured about 10 ticks by feeding dragons 32,800 food.
10 * 50,000 = ~500,000 xp
32,800 * x = ~500,000 xp
32,800 * 15 = 493,200 xp
Hence, using this method I estimate the xp value of feeding dragons to be 15 xp / food. Given the innacuracies of this procedure, it seems to reasonably back up an actual xp / food ration of 20 xp / food as referenced at the link in the top of this answer.