Yes there are a few select items such as gasoline and smokes that have enormous taxes built in but it is an exception, not the rule.
A 30% estimate is more reasonable (probably even high) because the rough estimate math works out that way. In
2001, the fed govt collected $2T, half was collected from individuals (page 34). People paid an average tax rate of
14.23% (income tax only). Add in another 10% for the taxes paid
directly to the feds (SS, medicare, etc) and that is 25%. That 25% makes up half of what the feds recieved. Those taxes are not hidden in any way. Of course this doesn't count state, city, county taxes that are paid directly but they are small in comparison (about 20% of what you pay to feds?).
There are also sales taxes, up to 9% in some places. It is collected by the merchant but you really can't call that a hidden tax because it's right there.
Overall, around 30% of what people made in 2001 covered roughly half of what the govt (local and fed) took in. For the most part, people spend all their money. Yes, a lot of that goes towards housing, insurance, car payments and investments but all of that has their own hidden taxes built in. For the most part, everything except for the cash people have that just sits around gets spent on something so it all gets taxed somehow.
Now I've rambled a bit, I hope it makes some sense, but we are basically left with 50% that the govt still needs to collect. They collected the first 50% by taking about 30% from us. How is it reasonable to say that they need to take nearly half of our remaining money to collect that other 50%?