I've just done 2 drops to flat bar conversions because of health problems.
On top of the price of the bars, parts that you (may) need to change are:
Handlebar stem - length and clamp size may need to change
Brake levers
Gear levers
Bar grips/tape
Brake and gear cables
Cross-check - I used the existing stem and tape from the old bars. I had V-brake straight bar levers in a spares box and all the cables and outer-casing.
On-one Jeff Bars, Planet X £30
Sun Race thumb-shifters, Spa Cycles £22 each
Henry Burton - I had thumb shifters, bars, stem and all cables
Shimano straight bar levers,
Wiggle - £12 each
Ergon Grips, Wiggle - £32.50