What percentage of a trade should be fees? For example, if I know that the fees for a trade would be €10 then what should I invest overall to make it worth while?
It depends on your return requirements. If you have a 20% return goal, and you expect the trade to have a 22% return, then the fees need to be no more than 2% of the total investment amount. (Note that if there's a fee for both ends of a transaction, e.g. a buy and a sell, then you need to double the transaction fees)
If you have no return requirements and do not know what a trade is expected to return, then there is no way to quantify a "reasonable" fee. Obviously, the lower the fee (in percentage terms) the better, since fees directly reduce your actual return.
If you want something to compare against, note that managed mutual funds generally charge at least 1% annually in fees, while passive funds (e.g. index funds) can have fees as low as 0.05%, but fees in the 0.1 to 0.3% range for index funds are more common.