It's a good practice to acknowledge everyone who applies for a job with your business. It maintains good will and saves you from getting lots of calls from applicants who want to know if you received their application or resume.
If you don't get a lot of inquiries, the cost of postage generally won't be so prohibitive that you can't afford to do it.
If you do get a lot of inquiries, you'll just have to decide whether the good
will you'll get from sending out acknowledgments is worth the cost.