I'm interested in buying Komplete 13 Standard. My question is: Is it possible to buy Komplete 13 Selected and then upgrade to Komplete 13 Standard, or do I have to buy Komplete 13 for full price?
That's perfectly possible, though I can't help wonder if the upgrade cycle wouldn't cost you more.
As for cheap: sometimes NI provides discounts for upgrading, that can provide an inexpensive way for this.
The cheapest may be to buy secondhand controller that comes with Komplete select. I have seen KK S25 for 100 USD... You get Komplete Select for lower price than buying it directly and as bonus controller which may or may not be useful.
And if you wait till Summer Sale (June I guess), upgrade to Komplete Standard will be most probably discounted. In past years it used to be 50%. It seems NI has changed its marketing and so it might be less. It was about 30% on Christmas Sale.