I do not think that this is a problem, this varies of course from one country to another according to the tax laws in each country, but I do not see a problem in declaring the last amount only in taxes as long as you say that you do not Remember what the cost basis in past years, I think it will be sufficient to declare about Only the last amount and pay the tax.
In different countries there are different taxes for cryptocurrencies. The number of people who use cryptocurrencies only increase and I think that the more countries will regulate Bitcoin and other cryptocurrency. This is an article what is a difference in tax rates for Bitcoin Taxes in different european countries
https://dailycoin.com/european-highest-lowest-bitcoin-taxes/In many countries it is nessecary to pay taxes for Bitcoin and other crypto.