From January 2015, this policy will work in reverse. In a complete 180 degree change to LinkedIn's policy, if you DO get a response (either a reply or a "not interested"), the InMail will be credited back to your account. If, by 90 days of using the InMail, you've had no response, the InMail credit won't be refunded.
LinkedIn says this change in policy is intended to reward success and provide a better experience for recipients. I actually think its intended to prevent spamming via InMail. Up until now there's been no downside for spammers whose InMails were ignored - the unsuccessful InMails were simply recredited. I suspect also that the majority of InMails get no response at all and, by reversing its policy, LinkedIn can generate more revenue. I don't have a problem with that - they're providing an excellent platform and deserve to turn a decent profit.
LinkedIn has various types of memberships or subscriptions - free, Job Seeker, Recruiter, Sales, Business - and each of those has a "plus" version. Depending on your subscription, you may be allocated a monthly allowance of inclusive InMails with the option to purchase additional InMails which currently cost around c£7 each. At that price, you'll need to ensure that they are well targeted and designed to generate a response.