You will get different ansewers from people depending where they are. But most of our countries share similar characteristics in our programs.
Your question has such a huge variation in answers. Some gymnasts will begin competing just a few months after they start gymnastics and other will take several years and many others will never compete but still continue to enjoy gymnastics for many years.
In Australia you can start competing from level 1, and competition is compulsary from level 4 in order to progress through the various levels. The majority of gymnasts will start competing in level 1, 2 or 3.
On average they spend a year at each level, but again this can vary hugely, some may be in a level for only a week, just long enough to test out and move onto the next level while others may stay on the same level for 3-4 years. But one year is the average.
Yes, to pass a level you need to compete on all 4 apparatus and recieve a minimum all around score. There are some gymnasts known as apparatus specialists and they may not do all the apparatus but this very rare at lower levels. Ultimatly the coach decides when you are ready to move up. Just because you have passed your current level, does not mean that you move up to the next level. This is a desicion your coach will make when they feel that you are ready to cope with the requirements of the next level.
Again, a competitive program costs will vary greatly depending on where you train. Lower level gymnasts generally train between 4-9 hours a week. By the mid levels you would be training 10-16 hours a week and the higher levels will generally train 16-25 hours a week. At the top level you will train 25-40 hours a week. So the cost will vary greatly. It is not a cheap sport because of the number of hours required, the low coach to athlete ratio and the cost of maintaining the expensive equipment.