Hey there, when I first started collecting, I was an out-of-control pony spender like yourself, and actually, it took me quite a while to rein it in. I just saw it as I needed to get the ponies I wanted ASAP before others snatched them up, but the more I've been around (will be 5 years this April) I realize that's not the case at all and there really has never been a shortage of ponies. There will be ones that are hard to find for a while, but if you're patient, you can always find who you need.
That being said, here's a thread with some of the awesome responses I got to help me with my new pony budget:
http://www.mlptp.net/forums/mlp-dis...g-pony-spending-bill-paying-money-saving.html
Some of the great tips I learned from those people are that you should try to buy ponies with just the money you have in your paypal account, so if you are a seller, this is a good way to go. Of course this doesn't help if you only buy in-store ponies...
And I know those don't really answer your question, so I'll also say this:
It depends on your income, and how comfortable you are with your personal finances.
However, that being said, if you're not that good with money and aren't sure how much should go where, I'd follow this little equation:
A general rule is that income should be broken up like so:
This won't help though, unless you set up a budget for yourself so that you know what your wants and needs are. If you spend 30% of your income all on ponies, it won't cut it because you know there will be more wants- be it eating out for dinner or a new pair of shoes.
What I did in the beginning was set myself a "pony budget" for each month. It was $40 per month, and at one time I raised that to $50. I've definitely overspent my budget, but I've gotten better about it and now I'm at a point where I don't even need a separate pony budget (partly because I have so many now) and partly because I've learned to rein in my wants.) I will say there are times where I've spent over $100 a month on pony stuff... yep, I felt bad afterward, but I still really enjoyed my purchases. I've cut that way of spending out of my life because it's not sustainable, but don't feel too bad. Just try to set a goal for yourself and reach a happy medium that works with your lifestyle.
Good luck!
As long as there is money for bills and food don't feel bad and enjoy your new ponies 
