Maybe I can help?
I'll list what I paid for each of mine (and possibly give myself a few nightmares in the process lol!) Also I bought in a few different currencies and since I'm working (and slightly lazy!) I'll leave a little of the work currency converting to yourself..Sorry!
January Carnation; $68 usd
Feb Violet;$200 USD but she was a trade
March daffodil ; $180 usd (still a purchase in progress!)
April Daisy ; $60 usd- a trade for nbbe heart throb (this girl was quite flawed)
May Lily ; £95 GBP
June Rose ; she had a mane cut and was $70 usd
July water Lily ; hmm...if I recall correctly she was $80 usd
August poppy; £99 GBP
Sept morning Glory; £110 GBP I think
October cosmos ; 600 DKK
November; £95 I think?
December holly; 600 DKK
I got a bargain on some of these! So don't take my prices too religiously, although if you wait you might get some nice prices
PKW is pretty spot on and I would put most of them at around $100-150 at the moment, depending on month, Feb and March are higher because they are harder to find. You would be lucky to get either of them for less that $170.
I was told by a few people that february and august are the most rare, but personally for me March was the rarest. In my search I found 4x August, and I got the first February I spotted but have found another since. March was a stroke of luck and there is another girl here looking for her also to complete the 12.
To be honest I think the rarest girl changes with the season! Years ago when bitthflowers were 3x their current value and I could never afford them , quite a few March came up for sale, but Febuary never came up. I remember so well because, it was long before my son's were born but my daughter were Feb and March babies so I really wanted these two, a few times I tried to get the cash together but $400 odd for a pony was far too unrealistic for me financially at that time. I'm so happy to finally have these girls and soon hopefully have March too, so I'm spamming brags a bit too often!
best of luck with your search!