Well according to Michael Gouby, there are 5 known F103's compared to 7 known F90's. As you say, Freeman has the F90 at R18, but classes the F103 at R19.
Personally, I do think Gouby has a slight tendency to estimate the number of admittedly very rare coins, on the low side of actuality.
As to whether you pay £5k for a fair example of a 103, well personally I wouldn't. The best known specimen is EF, and Gouby has that at £1200 (2009 values). A VF specimen of a 103 went at the Bamford auction in 2006, for £1000 - it can be seen here
£5k might be in the ballpark for a fair 1877 narrow date F90, though, as it has a lot more cache than the F103.