Ok so I've become aware that I need to solve for the constant of proportionality k.
dp/dt = k(20-p)
The price rises $5 in 6 months, so dp/dt=5/6 when p=10
5/6=k(20-10)
k=1/12
dp/dt=(20-p)/12
From here, I get what appears to be a faulty equation:
p = 20 - 10e^t/12
The reason I say this is...