Apple specify that the iPad battery should retain 80% of its original capacity after 1000 full charge/discharge cycles (NB, by contrast, iPhone is “only” rated at 80% after 500 full charge/discharge cycles).
Your ipad mini2 with A7 processor is still very usable, however, you should consider that it is unlikely to receive much (if any) ongoing security updates. Whilst the A7-based devices still have reasonable utility, you might not want to invest good money on a hardware platform that is already 5 years old; cash spent on replacing the battery might be better invested on a newer - more powerful - iPad model.
The cost of an official battery replacement for your mini2 will be going-on towards a third of the cost of the current entry-level iPad - which will seriously outperform your iPad mini2.
Hope this helps with your thought processes.