Having spent a few pennies over Christmas I had to get back to my budget for the clothes and shoes I know I'll need this year (sometimes need means want if I'm honest!) I have a few rules of thumb that I use for how much I'll spend on lingerie and this helps me figure out my lingerie budget for the year.
Everyday Bras Budget: To get the longest life out of everyday bras they should be worn once every three days maximum. This gives the elastic time to recover between wears. If you wear a bra once every three days it should last about eight months (longer if you're wearing it less often and caring for it correctly) which is 80 wears. I average between $0.80 and $1 per wear so I'm willing to pay between $64 and $80 for an everyday bra.
Sports Bras Budget: I wouldn't run in cheap heels so I don't run in a cheap plunge bra. A sports bra should work in ways that regular bras simply can't - disperse impact, reduce bounce and wick away sweat. I know the damage that can be caused by wearing improper support so I think sports bras are worth up to $1.50 per wear.
I average that I'll use a sports bra twice a week (if you're more active you need more than one sports bra so that they have time to recover between wears). I expect a sports bra to last at least eight months (preferably 12 if I look after it correctly) so in eight months that's 64 wears at $1.50 per wear means I'm willing to spend up to $96 for a sports bra.
Happy Bras (AKA Pretty/Must Have It/Luxury Bras): I don't think these bras are optional as they make me happy, I keep them for quite a while and life's too short not to wear beautiful things! I buy at least two seasonal bras every season in pretty colours or patterns that inspire me for the time of year. I'll wear them on and off for at least a year but probably more intensively at the beginning.
I budget for 10 bras a year that I don't rely on for my daily support or activities but that make me happy, excited about being busty and inspire me to dress up. For this I budget $75 per item and this is simply based on what it's worth to me to have these options. That's $750 per year.
Sales Budget: I expect to buy some of these bras in the sale and average savings of about 20% over the year. I buy my everyday bras as and when I need them because I refuse to struggle and make do; my breasts and sanity are worth more. But for some of my Happy Bras I'll wait for sales (like the huge End of Year Sale at Butterfly Collection that ends on Thursday!)
UPDATE: I should clarify that this is how I work out my bra budget but your own can be worked out using similar ideas. You NEED 3-4 everyday bras to get you through a year plus 1-2 sports bras if you are active. That's a basic wardrobe which costs between $200 and $300 per year (around $20-25 a month) Figuring out what kids of bras you need and then which bras you would like to have gives you the basis for your budget xx