Some basic maths nicked from Landyzone - of course maths found on the internet is highly trustworthy!

Taking your average family runabout as an example, the Bugatti Veyron has a drag coefficient of 0.36 and a frontal area of 2.07m^2 (according to Wikipedia).

At 250mph (i.e. 111.76m/s) the aerodynamic drag will therefore be:
0.5 * 1.23 * 0.36 * 2.07 * 111.76^2
Assuming an air density of 1.23kg/m^3

That comes out at 5724N, which translates to a drag power of;
5724 * 111.76 / 1000 = 639.7kW

Assuming a driveline efficiency of 94%, that’s equivalent to an engine power requirement of;
639.7 / 0.94 = 680.6kW or 913bhp.

If Bugatti wanted to increase Vmax by (say) just 10mph, from 250 to 260mph, the new engine power requirement is:
0.5 * 1.23 * 0.36 * 2.07 * 116.23^3 / (1000*0.94) = 765.6kW or 1027bhp

So the extra 10mph on top speed would require an additional 114bhp from the engine!

Phil

Note - This ignores rolling resistance drag (generally taken as around 1.5% of the vehicle weight) because this is insignificant at high vehicle speeds compared with aerodynamic drag – though it is significant at low vehicle speeds.