Social insurance, health insurance and income tax will be around 25% on €15k annual salary in Czech republic. And that's assuming they have no children. With children it's even less.

source: http://www.penize.cz/kalkulacky/vypocet-ciste-mzdy

Supergross salary rings a bell?

15000*27/12 = 33750CZK/month = 42.5% taxation

Add VAT 21%, petrol tax 52%...

Except the VAT and petrol tax have nothing to do with income tax. So you can't just add it to the ~40% income tax. 21% VAT is quite normal in the EU, so that's nothing special.

Also source on that petrol tax? According to http://uk.reuters.com/article/2014/07/23/uk-czech-tax-idUKKB... it seems to be one of the lower in the EU, as the cut would have "put Czech petrol taxes below minimum rates allowed in the European Union"