Working on a formula to ballpark rails projects:

estimating rails

N = number of models
D = developer days/model = I'd approximate 3
C = complexity factor = 0.5 (ie, implementing 2 models is 3x as complex as one model)
M = number of views / model = 4 (on average)
E = developer days/view = I'd approximate 2 (given time for revisions, etc)

x = "Complexity" = C * (N ^ 2)
y = "Code" = D * N
z = "User Interface" = N * M * E
total = x + y + z
= C * (N ^ 2) + D * N + N * M * E
= N * (C * N + D + M * E)

Then guessing on my values with a four model application:

= 4 * (0.5 * 4 + 3 + 4 * 2) = 32 developer days

Or, getting more complex with 8 models...

= 8 * (0.5 * 8 + 3 + 4 * 2) = 120 developer days

Or really large with 16 models...

= 16 * (0.5 * 16 + 3 + 4 * 2) = 304 developer days