Viable alternative for expensive MLCC capacitor?
You could use a solid polymer (low ESR) capacitor, a low-ESR aluminum capacitor or parallel MLCC capacitors of lower voltage and/or capacitance. Probably film caps with MLCC in parallel too, but they're not so viable these days.  

Keep in mind that the voltage coefficient of capacitance is quite large (and negative) with MLCC caps so using them near the rated voltage will result in their capacitance value typically being much less than the nominal value with a small DC bias. Similarly, the inductor has to be chosen so that it's not close to saturation. 



As an example, suppose you tried to use two CL31B475KBHNNNE Samsung 4.7uF/50V caps in parallel (about 7.5 cents USD each). The capacitance at 30V will be nowhere near 10uF.... 



That's probably why the datasheet specifies 100V rating. 

There are DIY LISN designs out there if you're concerned about pre-compliance testing for conducted noise. Note that particular DC-DC has an unusually low switching frequency, which may be of benefit, depending on which standards you are trying to meet. Testing is a must in any case.