I never found the cost of mx too bad when i raced.(4/5 years ago). Best thing you can do is go alon to a meet, have a mingle and speak to the local shop. Trails and motocross news has some bargains in the back, but you have to know what your looking for. I found buying a new bike (not the cheapest i know) every year worked out well as you had the spares kit and didn't have to replace hardly anything apart from tyres and the odd chain and sprocket in a season. Year old bikes always sell for good money too
All i can suggest is never skimp on kit!! good helmet, boots and armour, doesn't matter about the fancy colours, just go for the best you can afford. Great sport at all levels, freindly helpful people everywhere and most of all FUN!!!! Kinda wish i had good knees and wasnt so fat now... lol