First and foremost, what you are doing contravenes the Wireless Telegraphy Acy. I.e. it is illegal.
Secondly, the amount of money you mention is not going to improve your set up very much.
Metal quality will make little diffrerence to the received signal. What matters most is line-of-sight between you and the transmitter and at VHF this is critcal. Getting a good antenna as high in the air as possible helps, but may not help if your location is wrong. Transmissions you are listening to do not necessarily come from an airfield. London Control transmissions are broadcast from various stations around the country. If you are out of range of them then you won't hear them.
There are several off-the-shelf magazines devoted to your interest so maybe you should consult them.