Bird Dog fee questions

[Holly Jewell]

Hello, I am interested in how Bird Dog Finder's Fees are traditionally calculated. I found a 3/2 in Palm Bay 5 years ago and negotiated the amazing purchase price of $40K and it came with tenants! I didn't have the cash to buy it myself so I brought in a relative to be half owner. I closed on the deal and have managed it and overseen improvements over the past 5 years. My partner lives out of town and hasn't even seen the inside of the home but has been happy to spend 1/2 the rental income available. When she takes a draw, I always draw matching funds so that we both own 50% of what is in the investment home account. So we each have drawn approx $12K from the property to date.  

As the property came with tenants in it, it was my intention to keep it as a rental income property. My cousin keeps needling me to sell it and every time I tell her I will buy her out but she has never agreed to sit down and work it out. Lately, after we had a fight about another issue, she threatened to dissolve our partnership and force a sale on the property - knowing I have offered to buy her out!

Well I recently was able to convinced her to cool her jets and let my husband and I buy her out. As part of this buy-out deal I want to charge her for my 5 years of property management and oversight of improvements as well as a finder's fee for bringing her in on the deal. As you can image our $40K investment is now worth $100K so my partner stands to gain substantially for such a small investment by forcing me to buy her out at market value. 

She does not seem adverse to paying a finder's fee but we didn't sign any contract when we purchased the home specifying terms of a buyout. Can I charge her the a 10-15% finder's fee based on the value gained? i.e the difference between $40K and the resale value of 100K? I want to avoid basing my finder's fee on the purchase price as that would punish me for buying low... 

Thanks for any advice you can offer on the subject. :-) 

Sincerely Distressed Buyer