What are good coat enhancing supplements that aren't too high in cost?

Yup you are totally correct, a healthy coat IS a good one! It will shine, protect your mare from weather, keep her clean etc :-)

My gelding has sensitive skin so I give him a flax seed supplement. I buy Omega Horseshine - and I have found it to be very helpful. It is just a half a cup added into his daily grain ration once a day. It is not particularly expensive. I also take a flax seed supplement myself to make my hair pretty and shiny too :-)

I swear by Manna Pro Simply Flax. About $20 a month with horse.com's auto ship. It grew out my gelding's mane and tail and made his coat incredibly shiny!

I have had good results with wheat germ oil, which costs about $16 a gallon and lasts about a month with twice-daily 2-oz servings.

I tried Omega Horseshine because it was highly recommended, but my normally UN-finicky horse refused to eat it!

Manna Pro has a product called Show Glow that is awesome.

SmartShine by SmartPak. :) I use this over the spring/summer for my gelding (and take him off it during the winter and fall), and his coat has been complimented by at least 5 different people as being the healthiest, shiniest, coppery-est coat they have ever seen.

Regular vegetable oil will do the trick!