Your question needs to be clarified.

The actual cost of a passive radon measurement campaign depends on many factors but basically on your relation with the dealers so I have no suggestion about this;

However, E-perm system is available also in a version usable for 3-7 days instead of a minimum of 30 days of trace detectors such as CR-39 (e.g. Radosys Ltd).

The long term version of E-perm  claims to be usable at higher doses (integration time*concentration ) than trace detectors but I have no direct proof about this.

The e-perm system is theoretically subject to discharge caused by strong electric fields and other environmental factors. Moreover once you read an e-perm the dose value is lost while a trace detector can be stored (in principle) indefinitely for future recording.

Another non negligible issue is that trace detectors are lighter and smaller than e-perm which is of some relevance if you have to deploy  a large number of detectors in some environments.

I started annual radon monitoring in an important road tunnel in year 2000  with e-perm and swithched to trace detectors in 2003

The answer to your question could be different depending on your goal.

Devices based on electret (such as E-Perm, which is a commercial name) could be more/less appropriate (i.e. more/less cost-effective) than other radon concentration measuring devices depending on your goal, on the number of  measurements to be carried out, and so on.

Some comparative analyses can be found in several papers and reports, i.e. the IAEA report "National and regional surveys of radon concentration in dwellings: review of methodology and measurement techniques", IAEA Analytical Quality in Nuclear Applications No. IAEA/AQ/33, which can be downloaded at http://www-pub.iaea.org/books/IAEABooks/10646/National-and-Regional-Surveys-of-Radon-Concentration-in-Dwellings-Review-of-Methodology-and-Measurement-Techniques)

Well, Nowaday, RAD7 is good tool for radon monitoring in short term or long term. However, humidity factor is one of the problem. You can check comparatively explanation in the past:

https://www.aarst.org/proceedings/2000/2000_13_E-Perm_Passive_Electret_Chambers_Measuring_Radon_Thoran_in_Air_Radon_in_Water_Radon_Flux_from_Surface_Mill_Tail.pdf

There are many methods for measurements. 

Active and passive 

for short and long times.

some of them are reialabl and others are sensitive. 

The samples that you measure can determine the usage of specific method.