No offence, but no major version upgrade, i.e. from 4.0 to 4.1 for US$53.00 seems too much. You must admit that your expenses were/are well paid by Steel Beasts Professional customers - by military professionals that are your main targeting group. Personal edition is just for general audience and not main scope of your business.
For the value of 4.1 upgrade we can buy full versions of other sim/games, e.g. IL-2 Tank Crew, Steel Fury, Panzer Elite, etc. etc.
Of course, that is just my personal opinion. IMHO, changing your marketing strategy lowering selling price of PE would significantly increase the quantity of sales leading to increased profit.