On the topic of cost - it can be as little as £40 - if your on a modified car policy it could be just the admin fee, however if its on a non mod policy, you will be looking at anywhere up to £80 per % bracket.
Ordinarily insurers bracket the bhp output as 25% or less - 26 to 50%, 51 - 75% and 76 + %
Most insurers will charge you about £80 for up to 25%, I found comparing sites, that up to 50% the average was £150, anything over 50% would tend to add hundreds of pounds, or refuse to insure you.
In response to the early questions "would I get caught" the answer is - you would have to be lucky not to! I am not replying to anyone individual here!
OK - park your car in a tree, claim a repair - probably not. but if you are involved in anything more than a small bump with another vehicle and your in trouble. A lot of modern cars these days have various protective functionality.
For example, the ECU will disable write capabilities until its been reset on the accident counter. therefore if you park your car in someone elses and cause a massive accident, plugging in your bluefin will potentially result in NO CHANGE to your ECU map. Also (not a policeman here) but I am sure that you could also be done (if found) of tampering with a crime scene, or for all intents and purposes "perverting the course of justice" on top of a possible Dangerous Driving, Driving without due care and attention, causing death by dangerous driving.... the list goes on!
Now, the other thing you probably dont know is that the ECU is not all Read and Write, there are portions of most ECU's that have a counter within them. You can remap 99% of the chips code, but there is more often than not a sector that is write only - i.e. everytime you put a map on, it adds one to the counter, then when you change it again - it adds one more, also markings with timestamp is common.
A quick check can easily obtain this counter information, and a quicker look still to the M.I.B Records would identify that you have not previously declared a remap of the ECU chip, therefore, it would show that you have deliberately interfered with it.
For a small knock for knock your right, you probably would get away with it, but for a bigger accident (and especially one were injury is reported - even your own!) you will be shooting yourself in the foot.
Now to get to the point of "what if I am caught" - at best you will be given a slap on the wrist - but when did this last happen? Lets just say your write off a Merc SLK - easily a £200k car if you buy the right one. You have a remap not declared, the insurer finds out - all of a sudden they are NOT paying out. Why should they pay £200k for your accident when you have not declared vital information to them? so then your stuck with a cancelled insurance policy (these NEVER go away - thats why insurers comparison sites ask "have you ever had insurance cancelled or been refused") so your then looking at the costs of the damage, and you decide to go out and buy the cheapest car you can find, bog basic - no mods, no fancy frills - ordinarily, you might have been charged £200 for the insurance, but now, as most insurers wont touch you, only the select few will have you, and you can guarantee, you will be paying 10x the price every year because of your dishonesty on one ocassion.
so, for the case of court, fines, possible prison sentence, CCJ's may also be used, refused insurance, elevated premiums, personal injury claims, lawyers costs, third parties car / repairs, criminal record....
Seriously... Need I go on?
At the end of the day, its an individuals choice what they decide to declare, but for the sake of an extra £40 for a few mods to be written on a piece of paper, or a single life changing moment, its not worth the risk. I feel very passionate (as may be told by the above) of those who flout the law and as a result cost dearly those who have suffered from the result. imagine this, would you want to be sat in court, having already gone through the above farce, now looking at the fact you are now being perused for £50,000.00 because you broke a lawyers leg, and they couldnt work for half a year? or worse still, you seriously debilitated an individual with a life changing injury? just remember this - tell your insurer the truth and they will pay out, if they try to wriggle out, you can take it to court and if favored you will force them to pay for the full half a million claim if thats what it costs! lie to them and they have no reason or obligation to stand by you, and most certainly wont - but could you find half a million now?