This report from Credit Suisse puts the value of the world's wealth at about 195 trillion US dollars in 2010, with an estimate that it will increase to about 315 trillion by 2015.
You could try to estimate it by looking not at taxes but at figures for Gross Domestic Product. Places like the CIA Factbook try to give GDP figures for every country. These are somewhat iffy at times because of things like underground economies, but they would give you some idea of how much in the way of goods and services is produced in the world each year.
But is that wealth? How do we account for things like quality of life? Or do intangibles like that even count as wealth?
I think actual numbers would be impossible to decide upon. the only way one could come close to estimating the wealth in the world would be to look at taxes (but not all countries require income tax), expenses, and account information of individuals. Unfortunately, two problems arise here: many people would not allow their income to be scrutinized by third-parties and a lot of money is accumulated and distributed "under the table."
Merrill Lynch World Wealth Report 2010 (available online) states global market capitalization is $47.9 trn. The combined global wealth of individuals with investable assets >$1 mn (excluding primary residence) is $39.0 trn. These numbers are likely a large fraction of the global total.