"Funds with an ESG mandate tend to be a bit more expensive than other funds, but the differences are not large. There are reasonable explanations for this pattern. Most ESG funds are not very large, so they are not able to benefit from the economies of scale found in funds with huge asset bases; as ESG funds gather more assets, more of them should be able to lower costs for shareholders.

Also, funds with an ESG or impact mandate usually have extra costs for shareholder advocacy and similar activities that aim to move companies in a positive direction… There are plenty of cheap ESG options for investors who seek them out, including a growing number of sustainable index funds and exchange-traded funds."

[COMMENTARY] There’s been little in-depth objective analysis comparing the costs of buying and holding ESG funds versus those of conventional funds. Morningstar has done a great service here in delving into them. This is an important article for all investors to review.
Are Sustainable Funds More Expensive? By David Kathman, March 17, 2017, Morningstar, USA.