Most of the sources below concentrate on local spot buying. National network television rates are often a little harder to find. Some generalized costs are displayed in SRDS TV & Cable. Marketer's Guide to Media gives figures that may help you estimate costs. Costs for ads on specific shows usually show up in articles in the broadcast trade press. For digital media costs, see the IAB Glossary for Digital Media Buying and Planning.
Several companies audit advertisers' use of media. These media usage audits let you see what a client or its competition has spent on advertising in different media.
For basic information on budgeting and math, consult the following books.