It seems a bit pricey but I reckon on paying about 8 euros for a 'pint' (half litre) of decent beer in Paris (and I only frequent the back street bar that the locals use, not the tourist places), so that would be 12 euros for a pint and a half. A 50% supplement for restaurant (rather than bar) prices probably isn't so unusual.
While such prices would be expensive for draught beers in the UK, many bars now charge around £4.50 for a 330ml ('half pint') can or bottle of decent beer, so that would equate to about €15.50 for a pint and a half.