If your slats are more than 4 inches apart, your mattress may sag into the gaps. Look for "mattress foundations" with more slats if you have a heavy foam bed.
Many modern bed frames (platform beds) do not require a box spring at all, which could save you more money in the long run. buy cheap box spring
Box Spring vs Platform Box: Shopping Advice - Bedrooms & More If your slats are more than 4 inches