The basic rule of thumb that a lot of people/landlords use is that your rent should be no more than 30% of your gross income. So if you're looking to rent an apartment that costs $3000/mo, you would ideally want to make $120k or more.

You can adjust this number upwards or downwards depending on your personal situation, but it's a good starting point. For example, if you have student loans you need to pay off you might want to adjust the maximum you are allowed to spend on rent downwards. Likewise, if you don't have many other expenses you might be willing to spend a bit more on rent than is generally recommended.

> Bonds are sold on the secondary market at market value but that is just between private investors right?

Let's say the market value of a government bond on the secondary market is a 4.5% yield. (If it's a bond that pays exactly 4.5% then it will cost 100. If it pays a higher or lower coupon then its market price will be adjusted so that it yields 4.5% when purchased)

If the government goes and offers to sell new bonds on the market at the par price of 100 and is only offering 4% interest, who is going to buy them? If you're an investor, it's much better for you to just buy the existing bonds on the market which give you a 4.5% return. If the government wants to sell new bonds it's going to have to offer a rate that is competitive with the bonds that are currently on the market.