There is no magic formula for premium rates between types . I tend to only deal at the higher value stuff now ( So £600k aircraft value upwards) .
Type is important - a 'routine' SEP type with easy to source parts & a known engine is always cheaper than a rare type - not only easier to get parts , but on a rare type if parts are expensive then its more likely ( for the same hull value ) it may be written off. = increased risk.
Make & model hours - there have been a rash of accidents where high hours ( ATPL types) have claims due to unfamiliarity on type ( example I was flying with a Virgin 747 Capt in a Cub once - but he kept rounding out at a few hundred feet as his mental picture told him that's what to do - instead of keep flying for another 150 ft !) Equally a ex Reds pilot who openly admitted flying a C42 was an entire new challenge to fast jets or phantoms onto a carrier ! It dates the conversation to a few years ago - but I'm sure you can see what I mean)
In your example comparing pilots is always a tough call - ages , hours on M&M and even which insurer you deal with all have an impact .
Lastly in simple terms - the cost - of every £100 you spend - maybe 10-25% goes to a broker ( if you use one) . Then the insurer has their costs ( staff / admin/ regulation etc ) , then they pay to 'reinsure' the risk - so at the end of the day the actual ' profit' is much smaller than most think .
After all this you have a claim . A 'simple' prop strike ( common claim) in an SEP is a headline cost of maybe 25K ( give or take complexity - engine out , shock load test' prop , re-install ) - but on top of that there is always a loss adjuster dealing with ( Mclarens are common & known & very good at it ) who charge a fee - so if a 'simple' properly damage claim is maybe to up £30k now - and what about the damaged runway - throw in a few K for that - so now maybe £35k …….
This is a simple claim - no one injured , no expensive aircraft written off , no fuel contaminating the soil etc . If our simple claim is a £1000 a year PA28 - then that's at least 35 other aircraft premium wiped out - in fact more than that as of the £1000 - £200 went to a broker ( like me ) - another £200 for insurer costs - so now its £600 left over - so in fact £35k is 58 other aircraft paying £1000 a year all used up .
How many aircraft are at your base ? - Possibly NOT 58 ? But a 'simple' & 'cheap' claim has wiped out your entire fields aircraft premium - maybe your & 2 other fields ……… so when a cirrus goes for a swim & its a £250k hull - how many aircraft is that ! ?
A look through the AAIB reports quickly adds up to a HUGE amount in cost every year for several years ( and they are just the reported ones - lots more go unpublished ) ……
Throw in a fatality or two ( of which there were several years of a bad run - one year I had 12 people die in aircraft just I looked after ) - and that's a few more million in legal costs and payments ….. and the numbers get even worse ….
So in summary yes its got to go up - insurers seemed to get themselves to a point in the last few years where they charged as little as they could get away with to get the business - knowing if a claim came in if they charged £1500 or £1000 it is the same result - a substantial loss
The hours flown view is interesting as your aircraft is insured 24 hours a day , 365 days a year - if it flies or doesn't its insured. Insurers have a 'minimum premium' they can charge to allow for this & cover their fixed costs - so flying less hours can be cheaper to a point - but.... we all know if you don't fly things corrode... pilots skills fade & judgement isn't as good . Pilots push their limits as they want stay current etc - so less hours flown can actually be MORE risk for the hours that are flown .
All this in routine flying - get into anything 'interesting' ( read higher risk) and its hard times in paying .