As long as you make the journey 18 times in a month then a monthly ticket would be worthwhile. It costs £363.30 compared to £21 for peak singles each day. However, there is a cheaper way.
Contra peak travel between Euston and Watford Junction is charged at off-peak rates as long as the journey is only on that line. To achieve that you need to break the interchange at Euston. There are three ways to do that: leave the Underground at Kings Cross St Pancras and walk to Euston (takes about 15 minutes); touch in on a bus (and get straight off) between leaving the Underground at Euston and touching in at Euston NR; or wait 40 minutes between touch out and back in again (20 when going NR-LU). If you can do one of those then you can get a zone 1-2 travelcard for £138.70 and your PAYG balance will be charged £1.80 for each single extension from zone 3 to Watford Junction. If you travel on 20 days in a month the extensions will cost £72.00 making the total cost £210.70 for the month.
If you don’t break the journey then the peak extension fare is £4.10 which is still a saving (£302.70 for a 20 day month). Touching in on a bus won’t cost anything with a travelcard and drivers are used to people getting on and off again.