I play on Insane difficulty and build two bases right in the beginning, usually the HQ in China and another base in Europe. After two months, I have four or five. Two "fighter" bases (usually China and Brasil) with three Saracens each, and two "team" bases (usually Europe and USA) with a Firebird and six soldiers. Every base has a radar, and I build further off-base radar sites so to cover as much of the mainland as possible.
This is extremely expensive, but the money keep flowing in. I am able to catch many ufos on radar and shoot them down or see them landed, so I have around 40 missions a month. This means:
- many alien objects retrieved from missions (usually Kerrblades and Plasma pistols), which can be sold for good money
- I sell most of captured UFOs (further 8-20 thousands per piece)
- the nations are happy, so I think they pay slightly more.