That's broadly the case, but there are a few subtle differences. A three year bachelors degree is typically 180 ECTS credits (ECTS being the European Credit Transfer and Accumulation System), and a postgraduate masters is typically 90 ECTS credits (in the UK at least - in some European countries, 120 ECTS credit masters are the norm). A four year undergraduate masters is considered to be a part of the 1st cycle (in Bologna Process terms), and is typically 240 ECTS credits, some 30-60 ECTS short of a bachelors plus a postgraduate masters.
Putting it a different way, we expect our MSc students to work over the summer (carrying out their dissertation projects, submitted in late September), whereas our MEng students finish with their final exams in June of their final year.
Regarding fees, at my place in 2013-14 the tuition fees for an MSc degree were set at £5,500 per year for a UK student studying a non-laboratory subject, which is considerably less than the £9,000 that you'd pay for each year of an undergraduate degree. Of course, the £9k tuition fees are paid upfront by a government-backed loan which may be written off after 25 years, whereas the main funding for postgraduate masters is through career development loans, which typically impose much harsher terms.