Don't try and dilute it if it doesn't say to do so! This type of product is rarely dilutable and you will get a milky white mess which simply won't work. I'm afraid the cost is unavoidable if you have a lot of tar to deal with. In the UK, this is fact of life and even the cheap and nasty petroleum products cost upwards of 30 per gallon and this will still have one of the smallest profit margins for most suppliers.
Bug removal doesn't require this type of product. If a water dilutable product suffices, count yourself lucky because the reality is that you do the have much tar present to remove.
For detailing products, you can look to dodo juice tarmalade or car pro tar-x. I'm afraid both will cost dramatically more than those mentioned and they cannot be diluted with water.