I would suggest that elasticity of demand is rather more complicated than even that.
Different clubs will have different elasticities. The key starting questions are what is the maximum and what the minimum for each club's attendance? Given ideal form, opponents, weather, loads riding on it, Derby game, etc, etc. How many could they get?
And for the worst case. How many diehards would be there after 15 straight defeats on a wet Thursday night with nowt riding on it against another team in poor form from miles away.
The difference between the two is the elasticity of demand. Assuming they were priced the same.
To measure the price elasticity you would need to repeat the same game under the same conditions with a range of different prices. Impossible. So trial and error is the only way. Basically, experiment with donation only and see what happens. As Swinton and others have done. But, unless you can repeat it, with identical variables, which you can't, you can't know for sure.