The trip planner on WMATA's website will calculate the fares for:
trip planner
Fares on metro are based on distance, so you'll need to know which station you will be traveling
to in order to calculate fares. Using Metro Center as an example, Glenmont to Metro Center during 'peak of the peak' is $4.75 each way. If you take a bus to Silver Spring, that's $1.50. The fare from Silver spring to metro center is $3.45, minus a 20-cent discount for the transfer. Thus 1.50 + 3.25 = 4.75. So, ultimately the same expense. My advice would be to avoid transfers if you can--waiting for your connection is time consuming. Some day's you'll get luck and make the connection smoothly with no wait. Other days you'll miss and have to wait ten or twenty minutes. Day after day, it adds up.
4.75 x 2 (round trip) = 9.50 x 20 work days per month (approximately) = $190/mnth
Of course, the cost to other downtown stations are different. Use the link above to calculate your own costs.
commuting costs are definitely a significant expense. I've known more than a few people who turned down jobs because of the commute expense, and many who've chosen to live in the city because the commute costs makes the suburbs just as expensive as the city.