Obviously the bike itself is going to be the largest portion of your budget. That doesn’t mean that you should spend all or even most of your budget on just the bicycle. The first big question to ask yourself is "How much are you willing to spend overall?" Once you have this number in mind move on to the next step.

budgeting for your ride

Imagine to yourself you are going to take a 5 mile bike ride; think about the scenery, the wind blowing across your face while the sun beats down. Can you smell the outdoors already? Good, now you can start thinking about some of the things you will want to have before you even walk out the front door. First, does 5 miles sound far? If so you will be spending a lot of time sitting on that saddle, you might want something with more padding or maybe investing in some nice bike shorts. If that 5 mile ride sounds far, how much farther does it sound if suddenly you get a flat and will need to walk home. Now think about what season or time of day you imagine your ride to be. If it is summer, how do you plan on staying cool and hydrated; likewise if this ride is during the winter or a chilly spring morning have you left room in the budget for gloves, armwarmers or even a headband?

flexing your budget and managing trade-offs

some things are obviously more important than others. Without a bike how would you even ride? Start by putting potential purchases into 3 caategories. 
1. Must Haves

2. 

3. Things you would like 

accessories are just as important

Helmet Graphic Here
The most important accessory by far is a helmet. Wearing a helmet can drastically decrease the chances of a head injury if you crash! 

Bottle and Hydration Pack Graphic Here
After a helmet staying hydrated is probably your next concern. There are lots of ways to carry water with you including mounting water bottles on your bike or using a hydration pack. Water bottles are the cheapest way to go. Many bike shops will throw a water bottle and cage in if you buy a new bike from them. If you are buying everything separate, water bottles cost $3-$20 each and water bottle cages (The part that holds the bottle on your bike) range from $5-$50. More expensive water bottles come with insulation and keep your drinks cold longer. No matter what kind of water bottle you purchase, make sure it is BPA free. More expensive bottle cages might weigh slightly less, but all of them will hold a water bottle. Wing style cages tend to be a little more secure than the older style single hoop aluminum holders. Over time aluminum might also oxidize slightly and leave residue on your water bottle.