Welcome to WebmasterWorld, always good to see a new nick :)
It's very difficult to give 'benchmarks' by industry as there are just so many factors involved, including what the company will or will not change on landing pages.
Take these two scenarios for a free product.
Company A wants the product offer on a page that uses their usual navigation. Their usual navigation offers many choices. Their landing page is clear what the product is about, but there's not a clear path to conversion. With so many options, it's difficult to funnel a visitor into a very specific action. Depending on the product brand, form requirements, etc, their conversion rate could be 0%-10%.
Company B is willing to remove all secondary navigation. They redesign the primary nav to allow visitors to get to their site, but the vast majority of the nav pertains to the download. They have a clear policy privacy, spyware policy, and a some sort of trusted seal. Each page has a clear call to action about the product, the path to conversion is clear. Depending on the product (and audience) their conversion could be 1%-20%.
Now, when you mix a keyword, an ad, a landing page which is an extension of an ad (or vice versa), the amount of factors which could be impeding conversions continue to mount.
This gets to the point about testing. Test the ads, keywords, landing pages, etc. Are you reaching your target market? Start to find formulas that work better than others, analyze why they work, use the benefits, get rid of the negatives, and start all over again. Only through systematic testing will you really know what sort of conversion rate you can expect.
Sorry I couldn't answer your question, there really is no 'industry standard' that I'd ever be comfortable saying without a LOT of qualifiers.