At the lowest costs it will take about 5-7 weeks in total to process (4 weeks to get the invitation, another 2 weeks to get the visa).

FYI regarding DIY (verbatim from the Russian Consulate's website):

<b>Attention:In order to streamline and improve the visa obtaining process as well as for security concerns the Russian visa section is no more processing visa applications by mail, starting from June 1, 2010.
In the meanwhile the applicants unable to be present in our visa section in person are strongly recommended to rely on the services provided by the local travel agencies <b/>

At least in New York the Consulate often has slower processing from May to September as well for individuals applying.

All in all, though, it is cost effective if you plan on going at least twice in a year, or plan on any flexibility in multible entry, or may need to leave on a moment's notice (in the past we were able to get same day tourist visas issued in about four hours--pricey but do-able; that is not readily available now, usually the fastest turn-around we can do is two business days).