Child support depends on your particular state's laws regarding child support and your unique facts.
Typically, the spouses' incomes are added together, a statutory percentage depending on the amount of combined income is taken for the amount of the income that is determined to be child support, and then the contribution amount per spouse is computed based on the proportionate contributions each spouse makes to the combined incomes. Therefore, if each spouse contributes 50% of the combined income, the amount of support imputed to each spouse is 50% of the child support amount. If one spouse contributes 90% and the other 10% to the income, the first spouse would be responsible for 90% of the support amount and the other for 10%.
Once each spouse's base support amount is determined, then time sharing becomes an issue. These computations really do vary from state to state. In some states, the custodial parent is entitled to receive support from the noncustodial parent. Period.
In states that use joint custody or time-sharing programs, however, the amount of support owed by the noncustodial spouse to the custodial spouse will be adjusted by the number of overnight stays the children have with the noncustodial parent. The closer that amount of overnight stays reaches 50% of the overnights per month, the less the noncustodial parent pays. If the parents each contribute 50% of income and the time with the children is split 50%, then, theoretically, no child support would be payable to either parent.
In other words, the amount of support your ex-boyfriend pays is totally dependent upon the statutes regarding child support in your state and the facts relating to your relative incomes and time-sharing situation. The bottom line is that what you are stating above may not be unreasonable based on the relevant law and your personal situation.
Nevertheless, it never hurts to check on things before you sign.
You might want to ask your attorney how your support amount is being calculated, based on the relevant law and your facts. You might also want to consult with another attorney in your area for a second opinion before you sign anything.