• coastful,
• castful,
• costfull,
• costafull,
• cosstful,
• coostful,
• costtful,
• cosful,
• costaful

• costlier,
• costumery,
• costing,
• costumed,
• costly.

The adjective form of "costful" is "costly".

The plural form of "costful" is "costfuls". However, it is worth noting that "costful" is not a commonly used word in English, and it may be more appropriate to use alternative terms such as "expensive" or "high-priced".

The singular form of the word "costful" is "costful".