Of the solutions available, can hotun be the cheapest?

The fitter has several options, besides hotun, that would satisfy the regulations.
All have their pro’s and con’s

He can run the pipework to an outside wall and discharge in a conspicuous and safe place over a suitable gully or soak away. – This is suitable if the installation is on an outside wall and there is a means of discharging in a safe and conspicuous manner. The installer is faced with having to drill through the fabric of the building, sleeve and seal the tube. (Number 3 above)
He can run to an internal tundish (with air break to drain) which will be located in a conspicuous but safe location and then out through the outside wall to discharge in a safe place (Number 5 above) – Same issues as point no.1.
He can run to an internal tundish (with air break to drain) and then to a device that can act as a (mechanical) odour trap (Numbers 2 & 3 above) and thereafter to connect to a local foul waste or soil pipe – This has become an accepted and viable method but it does require two products with all the associated fittings required to connect them and especially in an under-counter installation where space and height are critical, it can be problematical.
However you can now take advantage of RA Tech’s unique and innovative product, hotun. In ONE product you get a tundish with an inbuilt effective mechanical odour seal, it’s the most cost effective solution anyway!