I also do pastel portraits of animals. I think the secret of capturing the animals personality is to indroduce a subtle element of characature. Not so it looks like a cartoon but so that you exagerate the distinct characteristics of the individual animal and the breed. I did the horse just before he was put down, so I tried to give it a poetic effect of the horse plodding off to horsey heaven, giving a last farwell glance over his shoulder. All I can say is the owner was delighted with it, so thats good enough for me.
I normally charge £500, so I think £50 is much too cheap. If you do a picture for someone you can still keep the copyright and sell prints and cards of the same picture. With the prints you can get more money for each one by sighning them and calling them "limited edition". Decide how many you think you will sell, (e.g 50) and number each one 1/50, 2/50, until you get to 50/50. Obviously you dont need to print 50 all at once as long as you keep track of which number you are on! You could charge £50 for a limited edition print, or £75 if you put it in a frame. Hope this helps.