Well, I guess the cheapest are those where you buy a property or make a bank deposit. Assuming the property is at market price it is technically not a cost.
In Egypt you can get citizenship by buying a property for at least 300k USD, in Turkey for 400k USD.
In Egypt and Turkey you can also get it by making a bank deposit of 500k to a local bank. But for Egypt you have to convert to local currency hold it for three years and you earn no interest, which sounds like a very︀ bad deal given that the EGP is plummeting.
Then we have the Caribbean ones, where︁ I think Dominica, St Lucia and Antigua are still at 100k (plus lawyer and other︂ fees) to get citizenship, but they are planning on increasing the cost.
Vanuatu citizenship costs︃ 130k USD.
Comoros islands used to have the cheapest official citizenship by investment program at︄ 45k USD, but they have suspended it.
But then unofficially in many African countries with︅ some connections, it is possible to get citizenship for much less. The Lebanese community in︆ various African countries usually know the ropes around this.