I speak as someone who wants to see all the null sec blocks under continuous war in high sec.
Goes to show that you do not have the slightest clue what you are talking about. Let me illuminate your ignorance:
A few months ago, INIT attacked us among other things in low sec. In the process, we decided to war dec them because it would be cheaper for us in the long run than having to constantly repair our sec status due to low sec structure attacks. During this period, CONDI and other related alliances also chimed in on the war. During this entire time, both major alliances started to shed 1 man corps, which created a myriad of new wars. Since we declared war on INIT and corps left INIT, we also automatically declared war on these leaving corps, which in turn would lead to massive cost under such a sliding scheme without the attacker being able to do anything against it.
Or in other words: My knees are perfectly fine to jump up and hurt.
I assume that because anything else would be useless. It has to be expensive otherwise it would not have the deterring effect that the OP sees.
Instead of such a useless sliding scale on number of wars, which is easily and actually abusable, as demonstrated above, war cost should be dependent on the number of chars involved and it should be overall reversed: Instead of making wars more expensive the larger the target is, it should be the cheaper the bigger the target is. Furthermore, huge blobs should pay more to declare war against smaller groups (the following numbers are only for the sake of illustration):
If a 1000 chars strong group attacks a 10 chars strong group, they should pay 1B ISK
If a 10 chars strong group attacks a 1000 chars strong group, they should pay 10M ISK.
If a 1000 chars group alliance attacks a 1000 chars strong group, they pay 500M ISK.
If a 10 chars strong group declares war against a 10 chars strong group, they pay 10M ISK.
Taking risks against the odds should be rewarded, not punished.