At 100mW and a 'hobbyist' level design you will likely only get about 1/2 mile of reliable operation, probably more so in the 100s of feet... There are always exceptions to the rule, but when you can pickup up a professionally designed radio collar for about $150 I can't see it really as 'profitable' and 'reliable' as you believe... Yes, the sum of the parts is substantially less, but there is more to a finished product than just the parts...