Good way to promote and monetize free apps

HI,

I cant use SDKs as I use AI, which doesnt allow user code or incorporation of SDK (that i know of). Only ad networks I can use are ones that use URL based API, like mobfox and google ads. ALso use my own ad network Advertise on Android Mobiles Using RAF App Ads

Now I am getting around 300 installs/day of free apps.

With this using mainly google ads I am getting around £4/day

I want a way to increase this.

I know the apps a seller and people are interested as the paid version have sold quite a few.

To promote I use buysellads, tweets, mailing lists.

Left as it is at the mo with no active paid advertising im getting 300 installs per day.

Would advertising really work, is this the answer. I can spend say £150 per quarter, this is reinvesting.

Ive seen sites like crazymikeapss and sites that you got to pay to review your app or make a promo video, are these worth it, do they work?

So what would be my best strategy.

Any suggestions welcome.