CE marking is self-certification, so the minimum possible cost of applying the mark is little more than the cost of the label itself. However...
You're supposed to maintain a "technical file" which includes your justification for asserting that the product conforms to all applicable Directives. This contains schematics, PCB and mechanical drawings, the Bill of Materials and so on, plus the results of any compliance testing which has been carried out. Typically a company will contract out EMC and safety testing to a suitably accredited test house, and will keep their reports in the technical file as evidence of compliance.
It may not be too much of a stretch to say that a simple product is "safe" without external testing, though you'd need to at least have a copy of whatever the applicable safety standard is for your type of product. If, for example, you comply with voltage limits, clearances, earth bonding requirements and so on, you may be able to demonstrate compliance by reference to CAD data alone. On the other hand, it's very hard indeed to show that your product "must be" EMC compliant unless it's been formally tested. Most people who try, suffer an embarrassing failure when their product is actually tested.