is there any other way to make my connection secured ?
Google isn't just complaining about "security" (which could include a number of different topics), it is specifically targeting encryption. The fact that the connection between the client and server is unencrypted. It would normally only prompt with this if you are allowing users to login (submitting username/password) over an unencrypted connection.
Installing an SSL cert on your site (or using a front-end proxy like Cloudflare to handle the SLL) is the only way to encrypt the traffic to your site.
However, this isn't necessarily a "costly process" these days. Cloudflare have a "free" option and Let's Encrypt is a free Certificate Authority that many hosts support by default.