Perhaps a table would be the best way to look at things.
DR400 C172 C182RG PA28-235 PA28R R114 TB20
1 130kt Cruse yes no yes+ yes+ yes+ yes+ yes+
2 600NM range yes no yes+ yes+ yes+ yes+ yes+
3 15 GPH yes- yes - yes yes yes yes yes
4 Payload yes no yes+ yes+ yes+ yes+ yes+
5 Cost yes yes yes yes yes yes yes
6 Cost to run £ £ ££££ ££ £££ ££££ ££££
Note, The Robin can do the mission profile but it is at the limits of what it can do but is all round a the most flexable aircraft being by far the cheapest to run.
It is clear that the PA28R and C177 dont offer enough performance above the Robin or PA28-235 to be worth the money, it is only when you get to the C182RG, R114 & TB20 that the performance vs cost makes sense.