If you're looking for cheap hours building then you would be well advised to avoid the 'cross over' aircraft. The majority are pretty new and compared with older permit aircraft which cannot meet Section S very expensive.
Aircraft like the Zodiac 601 show the tendency - microlight versions typically are around £30k, SEP versions the low £20k and Jodels and similar range between £12k and £20k.
Additionally the swapping between SEP and microlight is not that straight forward and tends to be a one off activity. The difference in running costs between microlight and SEP are very small particularly compared with the capital costs.