As a first-order approximation of your tax rate, you can take your salary ($60k), subtract the married-filing-jointly standard deduction ($12,600), five exemptions ($20,000), and the 3% of your salary you're already putting into your 403(b) ($1,800), to get a rough taxable income of $25,600. If you itemize deductions or have other pre-tax payroll deductions (for health insurance, etc.), the number could be lower.

This would put you in the lower half of the 15% tax bracket (which goes from $18,451 to $74,900). You'll save 15¢ on taxes for every dollar you put in the 403(b) until you get your taxable income below $18,451, and 10¢ per dollar after that. This is only for federal taxes, state taxes vary and you'll have to estimate that yourself.