Taxes in Spain are paid in arrears, this year you will be paying tax for 2008. As a non resident you have to pay income tax (La Renta) even if you don't rent out your property. As you do rent then the amount you pay is deducted from the rental you receive.
Personally I wouldn't be happy if my agent was keeping 24% back from your rental. Paying tax is your responsibility not there's and I'd be concerned that I could loose that money if they went bankrupt or disappeared with the money.
You can deal with your own tax affairs, I'd suggest you read "You and the Law in Spain" and decide. I believe Amazon stock this or any of the large bookshops around.
As to the cost quoted for dealing with your tax affairs, I think it's steep. If you tell us the region your property is in I'm sure someone will be able to recommend a gestor who would be cheaper.
If the property is owned in joint names then each person named has to file a tax return, that can bump up the price.