Vandana
Less expensive is a loaded term. Jabber can be cloud or on-premise and can be bundled with licensing you pay once except SWSS. Spark comes with WebEx and is a subscription based license and can be viewed OPEX and provides other features like messaging. If your intent is to have just voice/video then Jabber most certainly is something you maybe licensed for already if you have CUCM licensing. On the other hand if some units within the company prefer team messaging Spark can be deployed and can be tied to UCM using hybrid services to provide voice/video functionality too.
So it does vary by customer/scenario and how you look at licensing spend capital vs opex etc
If you want a more robust set of telephony features like park/pickup/hold/transfer/conf, Jabber is the way to go today. Spark is a first messaging client and second a phone. Jabber is a IM/P client with a robust UC stack
Hope this helps
Srini