The two most expensive components of the columbarium are typically the roof and base. Thus the more niches you can fit between these two components, the lower your per niche cost will be. This implies that:
Two-sided niches are more cost efficient than a single-sided unit.
Higher columbaria with more rows of niches have a lower per niche cost than a smaller version with less rows.
Cylindrical-shaped columbaria without decorative domes and/or moldings can help reduce the cost per niche.