Webster's Revised Unabridged Dictionary
| APA | Webster's. (2011). costly. Retrieved September 23, 2026, from http://smartdefine.org/costly/definitions/1992057 |
| Chicago | Webster's. 2011. "costly" http://smartdefine.org/costly/definitions/1992057 (accessed September 23, 2026). |
| Harvard | Webster's 2011, costly, Smart Define, viewed 23 September, 2026, <http://smartdefine.org/costly/definitions/1992057>. |
| MLA | Webster's. "costly" 27 May 2011. Web. 23 September 2026. <http://smartdefine.org/costly/definitions/1992057> |