Lamps are a part of projector ownership and are expensive. This is true for all bright projectors which use traditional lamps.
For example, the original (BenQ branded) W1070 lamp is about $230.
Lamps are typically rated to at least 2,000 hours or more.
In perspective, you could watch about 1,000 two hour movies at the very least.
In low power mode, the lamp on the W1070 is rated to 6,000 hours, in high power mode, it's rated to 3,500 hours. You may see 4,000-5,000 hours realistically before you want to replace the lamp.
Let's go back to 2,000 hours though. At 1,000 movies, that would mean that each movie costs about 23 cents to enjoy. If you get double that lifetime from the lamp, then a movie is less than 12 cents to enjoy.
If you watched two movies outside, every week, then you should reasonably expect it to be 5+ years (or quite a bit longer) before you would be looking at replacing a lamp.
So, while lamps are expensive, replacements aren't frequent unless you are using your projector 6 hours a day. In which case 'frequent' may mean once every 1-2 years. Since you have indicated a lot less usage, you may never end up replacing the lamp. But, I suggest you do save up for a new lamp and have funds set aside for a new lamp should you need it. ALWAYS get a factory branded original lamp to replace the one in the projector.