at "cost price" or "what I paid for it"

I don't think you can. I have never heard this term used with this meaning. A google search took me to one place only - the Urban Dictionary ( not usually a good sign!). The one entry here was a mispelt example from a Latvian. This leads me to think that this is a mistranslation. In English, flat pricing ( or more commonly, charging a flat fee or a flat rate ) is where all customers are charged the same, regardless of how much of the service or product they use. One extra tip for nothing: Don't write 'smth'. The word 'smth' does not exist in English, even though Russian speakers seem to be very fond of it. Nobody else writes this and nobody else understands it. Just take another few seconds and write the word 'something'.

“Flat price” means that there’s no extra charges, no more to pay, that’s the price, and you’re not really expecting any negotiation on the price. So it has nothing to do with what you paid for it previously. If you wish to specifically refer to the price you paid for it previously, you would say “cost price”, meaning it’s the same as what it cost you, and you’re not making any profit on the sale.

Thanks to all for the great answers!