If you don't already have a household budget, this is a good time to start, with the trip being the incentive to stick to it. Microsoft Excel has basic templates which you can customize to match your situation. 

Not computer savvy?  Get a pencil and paper, write down what you spend each month and what you spend it on. Compare this to what you take in.  If you live within your means the number will be positive; if you don't it will be negative. 

This can be a real eye-opener!  The real purpose being to make you aware of your spending with the goal of gaining maximum enjoyment out of each dollar spent.  

In "How much will it cost?" you priced airfares, lodging, meals, transportation and entertainment.  Now you need to put all those numbers together with consideration regarding the number of people.  (That is, say airfare is $300, but there are four of you.  Then airfare is $1200.)   

If you're traveling as a family or as a small group, lodging will usually be less expensive if you rent an entire property. Airbnb and VRBO are two vendors of short term rentals. 

If you're traveling in a group of two or more, renting a car or using a ride share/taxi, will usually be less expensive than public transportation. 

So, now you know what you need (Trip Budget) and you know how much is normally left over at the end of each month.   You can calculate how long it will take to pay for your trip by dividing your trip budget by the amount you save each month.  For example, if your trip will cost $500 and you can put away $50/month, it will take you 10 months to pay for it.  

But wait!  Here's where it gets interesting.  Take a HARD look at your monthly budget and decide what you could give up without really missing it.  This single step can turn your dreams into reality.